It could be overheating issues. Do you have any other CPU or GPU-intensive programs running while you play the game? If you have any performance monitor programs that show you CPU or GPU temperature, you may want to check them while you run the game, and check to see if they have any "shutdown" functions. Some programs will shut down the computer if it runs too hot (above a specified threshold usually).
Do your fans get louder when you play the game? If so, you may want to try turning down some of the graphical options such as multisamples and map glow effect.
You can also press CTRL+ESC during game play to open up the task manager. In the process list you can see how much of the cpu the game is using. You can also check the performance tab to see if the cpu is working extremely hard.
In any case, please do post your last debug.err file.
Thanks.
