It could be a overheating issue.
Turn off your computer and open the case.
Clean out the inside of your case with a can of compressed air (don't use a vacuum cleaner). Especially blow the dust out of heatsinks and fans. Don't let the fans spin while you blow compressed air at them, hold them gently in place.
Remember to take anti-static precautions before you start to work inside the case. Touching a metal piece of the case should do it.