Help with lockups on myriad amount of games

I have a g4 Mx 440 SE and in C&C Generals, MOHAA, Black Hawk Down, Battlefield 1942, Unreal Tournement, Warcraft 3, and simcity 4 my comp just restarts out of nowhere. Without any warning. It just shuts down and it only does it during games! Can anyone help!?!?