i had kind of the same problem but with doom 3
but its because the video card cannot handle the games specs and if overheats and has to shut down in order to not damage the other components. so you might need to get a new video card.(not saying u have to go out and buy one) but if u choose to make sure its an up to date one preff the new Radeon 9700+ or higher, or neother highly new video card. and since u got certain slots available id stick with the AGP slot, CAUTION:make sure its not a PCI or PCI Express Slot. one that has an 3D accelerator. any more questions just reply