S.o.S Plz Help

Hello, I Have a E-Machines T2341, 2300amd, 1024 ram and a Radeon 9600 video card. It has a problem when playing games, it will work fine for 30-2hours then all of the sudden will) A. The screen will go Black but the lcd light on the Tower is still on and sounds like its running, must hold power button off to restart or ) B.It will Just Restart. It was doing the same thing on my Radeon 9000 card. I can surf the net, Watch movies, listen to music and this problem wont happen, ive Tryed reformating my Hd, no luck , switching monitors, No luck, It never gives an error log even with automatic restart unchecked, my comp temp is normal( it seems it is anyways) and it dosnt make any odd sounds, sometimes this wont happen for a week but then when it starts up again it dose it alot. I would bring it into a comp shop, but collage is eating all my $, i also spent hours searching your forums couldnt find a answer Plz Help tyvm ~Kanada~