NEED HELP

I got problem when playing 3D PC games. When playing the game, It will suddenly hang or freeze. Cant do anything even moving mouse cursor and alt-tab. But when I press the power button to shut down the PC (I already pre-set the power button to become shut down button), the PC can shut down normally by go out to the desktop then shut down as usual.

My graphic card don't have temperature sensor so I don't know the actual temperature but I think should not be overheat problem. Because:

1. The game will only hang when playing those PC standalone game like STALKER, Ghost Recon 2.
2. Playing Dota no problem.
3. Playing 3D online game like Granado Espada, LineageII which uses high graphic quality no problem.

So need any pro here to help me troubleshoot this problem.

Hope to heard from you all here soon.

Thanks ya~