Thanks for the tips. I've kind of suspected it was a heat-related issue. I forgot to mention that every once in a while, I will hear a couple beeps from what I assume is the mobo speaker (sound similar to the POST beeps). This only happens when I'm gaming, and is usually followed by a lock-up within 5-10 minutes.
However, the only temp sensor I can find is the one with Sisoft's Sandra program, and it only measures CPU temp. I also heard it is wildly innaccurate. My guess is that since I only get problems when playing a game that taxes my video card, then that is what is overheating. Is there some sort of utility I can use to measure the temp on my vid card?
Also, if it is a temp problem with the video card, what can I do to cool it down? I already have 2 side fans blowing directly on it, in addition to the dual fans that came mounted. I've tried actually removing the side of the case to let more air in, but it doesn't seem to help much.
Thanks again for the tips!
EDIT: I reverified some HL2 files after install, and it seemed to fix the lockup problems with that particular game. I've actually been playing for about 6 hours without any problems. I tried the Doom 3 demo again, however, and it locked up within 10 minutes every time.