Well..ive had my share of compmant failures over the years. Ive had a bad power supply, then replaced it, and that power supply was bad too. Luckily a multi tester can find that problem.
If youve run a memory test program and everything checks out, maybe your vid card is shooting the craps, or overheating. Or gets crappy when its hot. Bad connects or cracked traces can work good until they get warm. I just had my ti4200 shoot the craps. if it wasnt for the strange colors popping up just before it crashed, i would still be pulling my hair.