the fact that there are games like Far Cry 2 that can run for hours without any problem leads me to believe that, perhaps, it could be driver issues. If Far Cry 2 can run without problem I think its safe to say there's no any hardware problem, since a hardware problem will show itself regardless of what game it is, as long as its graphically intense
To comfrim this, try installing the old driver that came with the graphic card. See if any of the crash happens.
Also remember that before installing a different driver, always remove the current one and reboot.