most likely it is over heating and shutting itself down if the fan isnt working correctly. if would try msi afterburner to see what temps you are running as well as the fan speed and compare it to the specs of your card. Also updateing you graphics drivers is a good idea as well.
I am using HwMonitor and my current Graphic card temps are 45-50 (im only surfing right now)
Also, my drivers are all updated.