Thanks for the ideas, guys.
Overheating was my first guess, but everything seems to be fine. I'm not doing any OC'ing, and the air coming from the fans seem fairly cool when this happens.
I did find a SiS agp driver that I didn't have installed, although it wasn't called "miniport" or anything like that, just called SiS AGP driver, in fact it didn't even have a chipset name that it was for.
I also noticed this morning that the "agp aperature" in my bios was set to 128MB, and I changed it to 256MB (don't know if this makes a difference). Well, I did all of this during the AM, and I will just have to play some games tonight after work to see if this fixed anything.
If this doesn't work, anything else? Does this sound like it could be a psu problem? I went through a few reviews on this Allied 400w psu, and a few people found the same problem.
I'd love to get an Antec, but funds have been tight, since some crackhead managed to blast out my car window and steal my mp3 cd player last weekend, plus I just moved and had to go to a wireless network, which was not cheap, either.