what temp is your processor before it shuts down... game playing increases system temps dramatically, if your overclocked processor is running on a standard cooler you may be hitting the bios shut down temperatures pretty quickly.
Other than that I would check your voltages... that radeon will like a lot of juice as will an overclocked processor some even require voltage hikes in the bios to achieve stability.... more current produces more heat.

Although it could be a stability issue just with the processor as it may require a voltage hike to remain stable in these situations

Im obviously making these assumptions with the belief that you actually have decent air flow through your case.

Ive got clocked systems that are 100% stable until you play games on them for the above reasons

Take the sides of the case and blow a fan at it... if it remains stable you know its heat related if it still crashes I would examine your voltages... I would expect to see a low 5v line to your system if this is an issue