That CPU heatsink had some blockages in it. If the CPU was allowed to run at high temperatures for prolonged periods of time, it could've baked your thermal paste (the goop that sits between the CPU and heatsink) and made it ineffective.

Have you checked the temperatures of your CPU when the freezes occur? If they are relatively normal, than the thermal paste is likely not the issue and is more than likely aged hardware. You could try running memtest to see if there's an issue with your memory or try just running a single stick of RAM.

Hmmm.. well the paste did seem dry. But...

This is what sensors is reporting after 1 hour of uptime:

atk0110-acpi-0
Adapter: ACPI interface
Vcore Voltage: +0.91 V (min = +0.85 V, max = +1.60 V)
+3.3 Voltage: +3.39 V (min = +2.97 V, max = +3.63 V)
+5 Voltage: +5.07 V (min = +4.50 V, max = +5.50 V)
+12 Voltage: +12.51 V (min = +10.20 V, max = +13.80 V)
CPU FAN Speed: 1834 RPM (min = 600 RPM, max = 7200 RPM)
CHASSIS FAN Speed: 0 RPM (min = 600 RPM, max = 7200 RPM)
POWER FAN Speed: 0 RPM (min = 600 RPM, max = 7200 RPM)
CPU Temperature: +29.0°C (high = +60.0°C, crit = +95.0°C)
MB Temperature: +27.0°C (high = +55.0°C, crit = +95.0°C)

coretemp-isa-0000
Adapter: ISA adapter
Core 0: +27.0°C (high = +89.0°C, crit = +105.0°C)
Core 2: +30.0°C (high = +89.0°C, crit = +105.0°C)

I believe the reason I haven't had a freeze is because I left port2 active on my kvm switch. I mentioned earlier it seems to only be a problem when the display isn't active. (the PC that freezes is on port2)

It's the only PC on my switch that uses VGA, so since my last post, I tried plugging the monitor directly into the PC. Tested again - it didn't make a difference. But this last time I booted, I let it sit at the lightdm login for a while before switching back to port1....

And now at this moment... it suddenly rebooted. I must have been on port 1 for too long typing this post...

So about 1 hr 15 minutes uptime.

Temps seem perfectly fine, so I doubt it is a paste issue (although, this could be verified by running something that stresses the CPU and see if temperatures spike).

So, if you keep a monitor plugged in and active, you don't have any freezes? If so, it sounds like it could be a driver problem. It does seem this freeze is different than the freeze you had with the other video card in the system.

As I did a couple more tests, I've isolated it further. I just had 3 hours uptime. I turned the PC on and left the switch at port 2. With the video still connected directly bypassing the kvm switch. Though the monitor went to sleep after a while, there was no freeze until ~20 minutes after I flipped the kvm switch to port1. Now since the video was hooked directly, that stayed active. During the 3 hours, I woke up the monitor a few times and verified that everything was still operational.

So that tells me the problem originates when the connection between the keyboard and mouse is switched through the kvm box. That doesn't happen to the PC on port1 though.

I can try using the cables from port3. Also, I'm getting a new kvm shipped to me in a couple weeks. Strange problem.

As for the driver... Tried earlier booting to Slackware-current and had the same result.

Is the mouse and keyboard USB or PS2? To try and isolate it further you could try and see if you have a mouse directly connected to the computer (and not connected through the KVM) if it craps out and the same with the keyboard.

And trying port 3 would also be a good test, because maybe things are going out on the second port. I will admit... it's been more than a decade since I dealt with a KVM and it was a 4-port PS2 KVM that my buddy used.

Too much heat for too long can cause permanent damage, which is what you may be up against here. The old PS, after cleaning, could be tried with a different motherboard, and vice versa, to narrow down the failure's location somewhat. After seeing the inside of your PS, I'd first try another PS on the motherboard. And, I'd try a PS tester on the old one before risking another motherboard of any value.

OP already responded stating he is not using the original power supply.

That's what happens when I get interrupted between reading and finishing typing - forgetting.

Happens all the time to me as well...

USB mouse and keyboard.

1. Trying port 3 did not work.

2. I left the computer up overnight (no switching) When I checked it this morning, there was an uptime of 13 hours and it was still running.

3. I then unplugged the USB cable from the PC (one cable that handles mouse and keyboard coming from kvm switch)

4. I plugged in the keyboard.

5. 3 hours later, still running. I unplugged the keyboard. Instant freeze.

I had to ask myself why suddenly it was instant. So I formed a theory that for the freeze to happen, the keyboard removal had to happen when the display was in sleep mode.

6. I rebooted the computer. After 4 minutes I unplugged the keyboard and plugged it back in 3 times. No freeze.

7. After 20 minutes, I made sure the display was awake and unplugged the keyboard. All ok.

8. After another 20 minutes, I made sure the display was asleep and unplugged the keyboard. Instant freeze.

I mentioned earlier that when I set acpi=off in the kernel I didn't have any problem. In the BIOS, ACPI is set to Auto, and the other 2 options are to use S1 or S3. I wonder if changing that, or the APIC settings in the BIOS would have any effect.

In the meantime, I've logged into MATE and told it never to put the monitor to sleep. Normally when the computer boots it sits at a lightdm login screen and goes to sleep after ~20 minutes.

It should be noted that when I say the display is going to "sleep", it's not completely true. I'm using the same monitor with another computer with an HDMI connection. There's a button on the monitor that I can press to switch from the HDMI to the VGA connection (on the old PC). If the display from the old PC is asleep, the button just blanks and then puts me back to my HDMI connection.

So for the purposes of these tests, it doesn't matter whether the video is active or not, or if the monitor is showing what's on the first PC or the second PC.