I'll gladly help you fix it, but I doubt it's a problem in the game, as no-one else is getting it. Let's get some basic information about your installation and machine.
What version and checksum show on the main game menu?
If it only happens after you've been playing for a while, it may be an overheating problem. Please report the temperatures of your CPU and video (GPU) 1/ before starting the game up and 2/ after say 20 minutes' play. If your PC's vendor has given you no way of reading temperatures, Google for and install SPEEDFAN.
please post a dxdiag.txt (you can delete everything after the "Sound devices" section for brevity), settings.txt, system.log and error.log files (you'll need to rename the .log files to .txt to upload).