You seem to have enough power, have you monitored the temps of you CPU and graphics card?
CoreTemp for CPU and GPU-z for GPU are tools i use.
If not overheating then perhaps PSU is failing, look for a PSU tester or do it manually with a good voltmeter.
If the screen flickers and/or it turns black,you should try installing the latest drivers.
If that doesn´t work,don´t worry.
So many GPUs from the 78XX series have been shipped malfunctioning,therefore you should do a RMA,if you still have the warranty.
I know because I have a 7850 and works like a charm but a friend of mine also has one and he has the same issues as you and he´s going to RMA the card.