Could either be a weak PSU or the CPU getting too hot.
If your cpu is getting hot or you want to help protect your com or you want to know thw averange temp i suggest speedfan. it's a simple program that ells you about your coms temp and fan speed(it can also change the fan speed to help cool your system.
I had an overheating problem when playing halo and stuff like that

But it needs a bit or tweaking. Btu you should get in like 5 min.