We need more information order to help:
1. What are you doing when the computer freezes? When you say 'shut down', does it cycle into a reboot and start up again afterwards?
How about some system information:
1. Operating system.
2. How much RAM is installed?
3. Is it always the same game?
4. What did you do before this began? Update, install or uninstall> Participate in File Sharing?