make sure all the fans are working. if these arent heat crashes, sounds like a part is dying, probably psu, mobo, or cpu. only way to find out is swap a part, then stress test. could be two parts (blown psu could blow mobo), so if it doesnt work at first, start swapping 2 parts at a time...
heres what i've got:
shuttle xpc sn95g5 v3
x2 4200
x850 pro
1gb mushkin lvl 1 ddr 400 2-3-2-6
160gb
dvd burner
I'll probably load linux on it and keep my os.... let the buyer put on xp if they want. I'm thinking its worth around $600. whats your opinion?