Bullseye340 0 Newbie Poster

Hello there, I am having some problems with my comp and was wondering if anyone here could give ms some direction. First off i have a home built comp. Im running windows xp pro, 500w power supply, 1 gig of ram, 80 gig hd, an amd 64 3500+ processor, mother board is a GA-K8NSC-939 with the nForce3 250Gb chipset. Video card is a nvidia Geforce 6800 GT. The problem is that when i am playing some games like (Hitman-Bloodmoney, or TombRaider) my computer will after a few minutes shut down all video and freeze. my computer will still be running but i have to manually shut it down and restart it because my screen is blank. It also happened the other day playing medal of honor Allied assult (but i have been playing that game for years and that is the first time it has happend and it hasent happened since). I was able to play Tomb raider but had to put all of the settings on low. With Hitman i put everything on low and it happened about 5 or so minutes in....Can anyone help me please????
Thanks,
Bullseye