i need to ask a computer related question.

background:
i need some help resolving a problem i've been having for months that i've tried to resolve without success. around easter i went on vacation for a week or so, only to come back to a computer that would restart out of nowhere. it got gradually worse. at first, it seemed random at first. now, whenever i do something stressful to my computer (and sometimes things that are not stressful) it just locks completely.

*please note: i can change the way the computer acts by checking "automatically restart". if this is checked, when the system fails for whatever reason this is happening, it will just restart. if it isn't checked, the computer completely locks indefinitely*

things i've noticed and suspected:
i noticed that whenever i'd play a game that would stress my computer, it would lock so i suspected the graphics card. however, switching it out with another did not fix this. then i went into my bios's hardware monitor, and i could clearly see my cpu temperature rising steadily. so i went and applied more thermal gel, cleaned the fan and the heatsink out, and applied a box fan to the side of my case to try and keep the temp down. after this, it still did not fix my problem. to this day, i suspect either the mobo for giving false readings, or the cpu being shorted. (that happens sometimes, doesn't it?)

solutions i've tried:
switching ram (that was the first hunch i had, however both sticks of ram are good)
switching power supplies
applying new thermal paste to my cpu
switching hard drives
using new cables

solutions i haven't tried:
switching cpu
switching mobo

for educational purposes, i will list:
things i can run without freezing (all the time):
internet explorer (froze once or twice)
mozilla (froze once, just today)
nnnesterJ - nes emulator
cuteftp
winamp
photoshop (only froze one time ever)
macromedia flash (froze once)
aim
visualboy advance - gba emulator (froze a few times)
snes9x - snes emulator
fruityloops studio producer (froze when rendering a song)

things i definitely can't run:
counter-strike
battlefield 1942
zsnes - snes emulator
hitman 2
bomberman online
mame32k - arcade emulator
anything that trys to render a video for more than a minute or so

basically, when i hear my harddrive start to spin up, and it spins for about a minute and a half, it will all of a sudden lock and the hard drive will stop spinning. however, the hard drive seems to not be the problem, considering i've switched hard drives 3 times to try and fix the problem, and i've switched the cables also.

to avoid the problem, i've just been chatting, posting on forums, messing around in flash, photoshop, and fruityloops, and playing emulators. of course, i own all the games i have as roms.

can anyone provide a possible solution outside of switching my mobo or cpu? although that may be the culprit, i'll have to wait to attempt something like that.

Edit:

i suppose computer specs would help: shuttle av41p (mobo) 2.6ghz p4 500fsb (cpu) /w stock intel cpu fan+hsink 1.5gb pc2100 ddr (ram) 80gb maxtor (hd) radeon9200 se (vid card)