Mine does freeze up sometimes (randomly) as well. Even though its not always. Most possible reason is because:
1), You don't use OpenGL mode (in the video options)
2), Update your video drivers (if you have Nvidia / Gforce, check www.nvidia.com)
3), Could be your Video card is overheated, that it will force you to shutdown the game. Perhaps some coolers would be nice if this is the problem.