I was having the EXACT same symptoms. Not sure if this will be of any help,
but here goes.
Went on for about 2 months of random blue screens (no text, just blue or
black screen) and had to physically restart it. Replaced the hard drive,
then the video card, the power supply, then the memory, and still kept
happening. I checked event viewer and nothing was ever logged, there was
never any type of error file generated or error messages displayed. The
temperature was normal and other than the random black screen during online
games, everything behaved like normal.
Then I replaced the motherboard and the problem completely disappeared.
After a little research, I found out that there was some "incompatibility"
issues between the motherboard/video card/sound card combination (epox
motherboard/ATI video card/Sound Blaster live). It was fixed wehn I first
replaced the video card because I replaced it with another ATI card.
It's not documented by either of the 3 manufacturers and none accept
responsibilty,
but users have been complaining about it. So maybe it's some type of
hardware incompatibility you can check in to.
Good luck.