Prmetime,
I just got done fighting this myself. The system would work fine for a while, but then start giving me random guru's. What I ended up doing was pulling all of the fast ram out of the mb, cleaning the legs (used a small emory board), and put them back in. Also on my system, the ram speed was set to 16 instead of 25 (it's a 25 mhz machine). Don't know if that was a problem or not, but before I did it I couldn't run a single burn in loop successfully and after I did it I was able to run till I got bored with no problems.
Try the ram, see what that gets you.
Good luck!
98PaceCar