could be an overheating issue, install speedfan click here and click in the download section where it says "The latest version is SpeedFan 4.24.", this runs in the taskpane and allows you to monitor the temperatures in your system.
3d games push your proc temp up and it might be that your motherboard has a high limit at which the system shuts down to protect itself. look in the bios and see if you can set the cpu maximum temp to 65°C (for AMD procs) it might be set at 50° and its quite easy to reach that with some games.
hope all goes well