I experienced that back in the day, it was my video card over heating / not enough power.

Check to make sure your system isnt melting, and Id make sure your powersupply has enough juice.

I ran a system for about 2 years with major heat issues and just barely to little power.

I thought my card was fried, threw in an 800 watt powersupply for a new upgrade and was blown away when my little graphics card started performing 200x better.

Anyways just my personal experience with that, good luck.