I think that you possibly have some counterfeit RAM... My sisters ancient laptop did this when I put the Nexus hack with RPG mods and no clip. And it crashed when half of the area was in a post-apocalictic scene. Either, it is your RAM, or it may be your power supply, because nVidias cost you power, as well as your whole computer. Try to play GTA4 maxed out and play it to the limits, and if your computer shuts itself down by itself, it means you're getting some power supply that has 100 w more power.