Game Shuts down computer after 15 minutes

[r6456]

I played the game on steam for a total of 250 hours with no problems. About 3 or 4 months ago the game would cause the computer to crash after about 15 minutes, it would just go to a blank screen and I would need to reboot. I ask steam support for help and after several attempts to fix, I gave up. I really like the game and I purchased crossworlds hoping I would not have the same problem, however I do. I think other people are having the same problem and there is a solution, but I need help finding the solution.
Running Vista, NVIDIA GeForce 9400 gt. I downloaded the current dirver, updated direct x