What a pain…
I have had a rough time recently... My PC started acting up last week and I was never able to find the root of the problem. The first problem I noticed was that the system just powered itself down completely while I was processing some video from my camcorder. I rebooted the system and it came back up with no problems, but I immediately noticed that the video processing was running way too slow. The system ran for another day or two without abruptly powering down, but then the problem came back and got progressively worse. It came down to the point where it would shut down while sitting idle. I started trying to eliminate problem potentials at this point... I replaced the video card with no positive results. After that, I started disconnecting peripherals, swapping out RAM, and disconnecting and reconnecting hard drives with no luck at all. I finally decided to let someone else look at it for me.
The above photo shows the root of my problem. This plastic bracket holds the cooling fan in place above the microprocessor (Pentium 4 / 2.8 ghz). The plastic had gotten brittle and one of the support posts had broken, allowing the fan to shift positions above the CPU. This minor position shift was allowing the processor to overheat, which caused it to initiate its own thermal shutdown process.
I doubt I would have found this problem on my own. I could not see this particular part without removing the motherboard from the computer case. So... A couple bucks for the part and $75 for labor has me up and running again...