Can you post PC specs? I had a similar issue when installing my new R9 290 where PC would just crap out on me. I do believe it is your power supply as said above. I had a Corsair CX600M, and it only had two 8-pin connectors, and one was needed for something else, while the graphics card used the other. Sadly, you cannot draw a ton of power from just one cable, you need to have it coming from two separate cables, or else your graphics card wont be getting the power it needs. I ordered a Corsair RM850 (amazing power supply), and had plenty of 8-pin slots to use. I now run the graphics card with no problems whatsoever!

:EDIT:
Sorry, just read your reply above! I suggest buying a Corsair AX or RM series power supply (other models can be dodgy), SeaSonic, or Cooler Master V series power supplies, they're all very high quality. While the 770 certainly isn't a huge power drawer, I'd still buy a 750W+ PSU with at least an 80+ Gold rating.