After reading the three other posts in this forum pertaining to inability to chg pw I found nothing that is more than vaguely similar to the issue I'm having.
My Ubuntu 16.04 system has been rather difficult of late, causing me problems with password entries so that I must re-type the pw multiple times B4 it's accepted. This is especially the case in terminal. I'm not a perfect typist but not that bad either.
Recently it has come to my awareness that the Thunderbird email client probably has an exploit, and this morning I ended up completely un-installing it and installing Sylpheed, which so far seems okay. I will offer a detailed explanation of my experience with T-bird if it's requested of me.
Getting back to the PW issue -- Deciding to go ahead and change the root pw today I followed expert directions for using the shell method and root shell prompt, typing passwd username (i.e., passwd mike) and entering new password at prompt.
This was rather sketchy, as I entered the new PW at least 20 times, confirming it in each instance, only to be advised that there was an "authentication token manipulation error" and sulogin: input overrun at /dev/tty1t. Additionally, in many instances the password entry process was interrupted as though it were on a security-sensitive limited time frame so that the page I was on at the cmd prompt shifted, overlapping the preceding page - the one with all the options, such as "drop to root shell prompt." With a plethora of characters interposed and lines truncated within a miasma of varying red and black colors it was just about impossible to make heads or tails of what was going on. It was at that point each time when I pressed my laptop's power button and shutdown, then rebooted.
After about 20 tries interspersed with constant reboots I gave up and sought help. But it is inimical to reason and logic that the passwords I entered so many times were wrong in every instance.
As well, I noted that in typing, some of the characters among those I was able to see did not enter at all, and the entry results of those attempts were like a sick joke, reiterating 'misspelled words' that I'd typed correctly. So, if that was going on with the entries I could see, such as "assword ike," what about the typing of the invisible passwords?
It's not a keyboard error; the keyboard on this laptop is good.
It is obvious that something is interfering with my entries, adding, removing or in some way amending them to prevent me from effecting change of the root pw.
Gee. . . that is extremely reminiscent of Windows malware!
I believe my machine is infected. If this is the case, and I would bet on it, then there is no more for me to do but fresh-install Ubuntu and spend all day tomorrow coming back up to speed. Multiple deja-dup backups that I have stored on removable media are now relegated high-risk and must be discarded.
I'm a retiree age 72 and out of the rat race; so the foregoing is no big deal for me. But nobody likes to be had, and I am certainly not an exception to that rule.
Here's wishing all of you well and fair winds!

Are you trying to actually use a separate root user? Are you familiar with the "sudo" command in Ubuntu to gain root privileges. See the link below and explain whether the problem is with an actual root user you created or with using sudo as the primary user with administrative/root privileges.

https://help.ubuntu.com/community/RootSudo

Your other problems sounds like it is related to graphics drivers. What graphics card do you have? Have you tried to install another driver through the System Settings?

Two ways to go (after you consider sudo as above).

You can assume that your system is infected and do a clean install from known good sources. Yes, linux is not immune to problems.

You can also consider it a hardware issue. Still not proven to me that it isn't. May end up playing with this for a while. Any number of issues could be going on.

Guess it could be some fat finger, lazy palm, sliding key pokes.

Yancek and jefro

I do apologize for my tardiness in replying.

In re jefro's comment about doing clean install, this was done within 24 hours of last post (3-13-17) and things went okay for a while. But later after breaking down, lol and resorting to a deja-dup restore to regain all my lost pictures and documents, the malady reappeared.

Today I made another attempt at re-setting ROOT PW via grub menu, and it was an abject failure consistent with its predecessor.

Either the malware was transferred over from thumb drive during restore, or this laptop, which I bought new in 2014 from System 76 has an infected firmware. It is easy to covertly run a firmware overwrite. All it takes is, say, to reboot after a browsing session with a firmware iso hidden in the browser cache, so that the installer package runs during the next shutdown. When next reboot is done the 'upgrade,' which is most likely a piggy-back, write-protected onto its own private sector of the CMOS, will function without the user-victim's awareness.

Clam TK comes up clean except when I enable PUAs, and I don't rely on those false hits as being valid; but I will go back and re-run a full system scan with PUAs enabled and analyze the 54 or so possibles on the threat list.

We are all aware by now of the American Intelligence Community's long-term access to every operating system, including Linux. This does not bode well for our personal security -- and needless to say, it is ignominious to our personal dignity.

John McAfee of McAfee-Intel Security has suggested that a fix is in the making as per information provided recently by Julian Assange.

. . . Let us hope this is genuine and that it will effective put the scoundrels out of business.

In reply to yancek's questions: "Are you trying to actually use a separate root user? Are you familiar with the "sudo" command in Ubuntu to gain root privileges. See the link below and explain whether the problem is with an actual root user you created or with using sudo as the primary user with administrative/root privileges."

. . . The answers are no, I have created no secondary root user and yes, I am familiar with all the basics of the Ubuntu 16.04 system and then some. I know how to use terminal.

But you see, yancek, I'm not initially dealing with root when logging into grub. As you know, grub must be accessed by pressing the shift key during startup, then navigating to the page where a list of options for running system repairs is found. There, the user must scroll down to the line that says, 'Drop to root shell prompt option' and press enter.

Ostensibly, this leads to a command prompt where the user is able to change the root PW by typing, 'passwd user' (in my case the user being mike), entering new PW and repeating, after which the root PW should be changed. But in my case that isn't the result.

What's happening to me is. . . I'm getting preempted by another program before I'm able to finish typing the PW reiteration -- and whatever it is doing it then whisks me back to the previous page, the one listing all the maintenance/repair scripts that run as standard functions under Recovery mode.

Additionally, once this mysterious switch occurs and I'm back at the foregoing page, everything freezes and the only remaining option is to press the power button for a forced shutdown.

Additionally, yancek, you ask, "Your other problem sounds like it is related to graphics drivers. What graphics card do you have? Have you tried to install another driver through the System Settings?"

Ans.: The graphics card is a c.2014 PCI-e mini (don't know the name just off hand) that works without a hitch; I've cleaned it once recently by 'erasing' the gold pins with a rubber eraser to remove oxide buildup, but it has never caused any problems that I know of.

As to installing another driver through System Settings, I often check to see if such a driver exists; so far, no cigar. All it ever says is, "Using Processor microcode firmware for Intel CPUs from intel-microcode (proprietary)" and below that is ticked, "Do not use the device."

So actually, there is nothing given in the way of information as to WHAT DRIVER is used for the firmware!

. . . But SOMETHING is functioning there or my system's graphics would be full of artifacts.

One last word before retiring this verbose commentary: The old difficulty I'd had with typing has recurred, so that the cursor and the words behind it as far back as the previous period disappears suddenly, resetting the words just typed as an interjection that could be anywhere in the preceding message body. This forces me to hunt for the missing line so that I may select-all, then drag-and-drop same back where it belongs, as the most recent entry to the message. The malware is doing this as a harassment, since I am to be punished for writing ideological opinions which do not resonate with my controllers.

If anyone happens to be familiar with this sort of problem I would enjoy keeping my current system and kicking out the uninvited bums who have set up permanent camp in my computer.

But of course, we all know that as of now, anti-malware software for Linux is scarce as hens' teeth and that ClamTK is the only kid on the block.

. . . C'mon, John McAfee!

My thanks to yancek and jefro for your input.