Helo bai
I have read your post and it seems like we have thesame processor. My mobo is Nvidia650i Ultra. At first there is a problem with 45nm compatibility and updating the bios was the solution. Second problem is when i ran it with COD5 at all settings max it will shutdown. The solution is to go to system properties, advance and uncheck automatically restart at system failure. Now at COD5max, it will hang up! hehe! And im also sure the problem is not in the power supply because im using 800w. My VC is 7950GT.
After that, i tried to fix my CPU cooler put some grease and the problem is gone. Since we have the same processor, it is better to check your bios temp threshold and the temp of your cpu itself. More often the cause of shutdown its bec of temp. Try to check also if your mobo is capable at certain high temp since updating your bios can also change the capability of your mobo sometimes. Try to contact the manufacturer of your mobo i know they can help you like what NVIDIA have done to me.
Hope this helps.... best regard!