Check you powersupply.
But what you are explaining sound more like a temperature problem. Use GPUZ to check what your VRM's temps are... Make sure they dont go over 90c and you should see "throttling" stop.
I don't think my power supply is the problem. Its a 650W 80+ gold FSP power supply which should be more than sufficient.
And I have checked all temps with GPU-Z and nothing is above 90C.