It used to behave in a perfectly standard way. I switched on at the mains, then pressed the button on the front of the case. There was a specific noise (presumably the power unit fan starting up) and then the bios key list came up on the screen, followed by the bootloader. When I invoked shutdown, the machine switched off and stayed off.

Now I switch on at the mains and, a moment later, I hear it starting up with the usual noise, but without the switch being touched. Nothing happens for a moment (no lights or anything); then I hear the startup noise a second time, lights go on and off, and it boots normally. After shutdown, I have to switch off quickly at the mains, otherwise it switches on again for a cold reboot.

I can live with this but I'd like to know the reason for the changed behaviour.

At the moment I can only think of two possibilities.

Either some component in the power on circuit has failed or the BIOS may have an automatic power on feature when main power on is detected that is now enabled. The later might be due to the CMOS battery needing replacement although I would of expected the hardware clock to be reset which usually triggers a BIOS fault.

Clock runs normally.

About the only thing you can easily check other than BIOS settings is the power on switch with an ohmmeter to verify it is not normally closed all the time.

Surprizingly, a lot of interesting ideas from search of:
linux pc reboots after shutdown when power applied

Did you just do like: remove laptop-mode-tools pkg?
Try removing ethernet cable (wake on LAN).

I always highly like all posts by hazel!

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Useful search string! Better than the one I used before posting.
This computer isn't a laptop. It's an old HP desktop and I doubt if its bios has any fancy options like wake-on-lan. In fact I bet it can't hibernate at all.

Fortunately it depends on mains power, so it stays off when I switch it off at the wall. I only discovered that it would reboot by itself yesterday when I deliberately left it live to see what would happen. In a laptop, this kind of behaviour would be a nightmare, as you can't switch it off at the mains. On a desktop, it's not a big problem, as long as it doesn't mean that the machine is beginning to fail.
Thanks for the flattery!

When you say "mains" do you mean that you use a power strip where all power to the system can be removed?

Try this. Power down system. Remove AC plug. Press power button a few times. Then replace ac power plug. Does it act normally?

I got a dollar to a doughnut that the switch itself is broke, or, in two dollar phraseology, suffered a mechanical failure while in the on position.

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Tried that. It still started the same way: startup sounds, then silent, then startup sounds and normal boot. I guess Frankbell is right and the switch has failed. That's OK with me as long as it doesn't mean that the machine is beginning to break down.

PS Actually the new regime has its advantages. My dog is very jealous of my computer and often tries to nip me when I switch it on. To avoid the hassle, I have got into the habit of switching on when he isn't in the room. Now I don't have to switch on any more!

Another computer gone to the dogs!

(Sorry. Couldn't resist.)

I have a cat. What she does is ignore. Unless you look like a kibble.

Joking aside, I have observed that computers have become so reliable that, when there are problems, users tend not to think that they might be hardware related.

When I was in tech support (I worked for a manufacturer of card access control software and hardware for several years), I learned that the first thing to do, if hardware might be an issue, is to check the hardware, if only to rule it out, because no amount of mucking about with software will fix a bad cable.

Sound to me like the setting for what to do when power is cut (can't recall exactly what it's called) has changed. Basically you get something like "memory" which means if the computer was off it stays off and vice-versa, off and on.
I have a similar problem sometimes when using a laptop and the cat starts to get jealous and launches herself at my hands with all claws and teeth.

BIOS says "After power loss: Off". I take it that means that once you've switched off at the mains, it stays off until you press the button to switch it on again. I didn't mention this, but recently it's gone back to this normal behaviour most of the time.

Yes, that's right and the setting I would expect you want. Perhaps either a BIOS or BIOS battery issue meant it changed (going by your other thread) since it is now doing as it ought?
I also wonder whether jefro's thought process was correct but the procedure didn't work properly, for some reason. In other words some residual charge in a capacitor or such was causing the issue and just had to be reset. If it weren't for your other thread I would think it the most likely cause.

I'd start at the source 1st. My place is 3 phase power with 240 volt with a high leg.

125v on all household circuits. 180 volts on one leg < means 1 hot wire > that only goes to 3 phase equipment like central air conditioner electric motors and compressors.

What I am getting at. I'd take my fluke multimeter and read exact voltage on what you call the mains. Check for a voltage drop. Because old places have the screw terminals loosen up over time where the wire goes onto the plug. Part of a preventive maintenance thing I did as a electrician at a hospital was to pull the plugs out of the wall and check the tightness of screws for this very problem.

Because you did not want some one dying from a faulty mains/outlet.

If you have any electrician friends with a multimeter and a screw driver.
This is a easy check and procedure. Do not attempt this on your own. You can get killed if not turning off the right breaker or the neutral wire has some sort of hot voltage feedback. I used special tools like this before taking apart a outlet.

http://www.homedepot.com/p/Klein-Too...1SEN/100661787

Hi hazel,
I'm with frankbell: The power switch on the front of a PC is generally an instantaneous push to make switch. Provided the mains is providing power to the PSU (and any switch on the PSU is on) there is actually a part which is live and supplying power to the front panel switch so that the system can recognise when it has been pressed. It then powers up the main supply and the system boots.
I used to have an old Mac LC475 which had a similar system but rather than the psu supplying the startup voltage, there was a battery inside. Common fault: Bad battery, no "Bong, no boot!"

I'd be inclined to remove the switch and check it, you're possibly getting some sort of intermittent action as it fails. I had a system where the switch stuck in so it was continuously trying to power the computer up. Severe smell of nasty burning plastic and motherboard before I found it. (I'd left the system on while I went out. Not a good idea in this instance.)

Check it out and replace it if necessary - Be safe!

Play Bonny!