Another thing to consider is the possibility that your PSU does not put out enough wattage to handle your GPU and CPU under heavy load, and that's what's causing your PC to power down.

For what you have in your case, I'd recommend at least a 750 watt PSU, preferably 1000.

And never underestimate the effect dust buildup and poor cable management has on your components! Always keep your case free of dust, and be mindful of airflow when positioning cables. The more open space there is in your case the better.