A couple of possible issues...
1. You've got a virus
2. Your power supply is failing or underpowered
3. Your computer has it in for you
4. You may have driver compatibility problems (heck, they get blamed for everything, don't they)
Does the reboot happen at the same time and place? Consistent behavior makes it easy to resolve problems.
You might also eliminate as many other programs as possible when you're playing. Use Task Manager to stop all non-essential applications.