Grey screen of death?

Okay so every once in a while my computer screen turns grey and i can't see anything but i can still hear. This only happens when im doing something graphically intensive like playing a game or rendering video. I'm pretty sure its my video card (obviously) because everything else keeps working and i can run prime95 for an hour without a problem but 10 minutes of assassins creed 3 and i gray screen. The only way i can fix this is by restarting. It only started happening after I got my new case and moved everything into it so I may have forgotten to do something when swapping but ive checked many times and cant find anything. My best guess would be my graphics card overheating but that doesnt make any sense to me.


Specs
CPU: AMD FX6100 stock
Graphics: Sapphire Radeon 6850
PSU: Corsiar cx500
Hard Drives: 750 seagate barracuda and 540 gig hard drive i took out of my laptop
Motherboard: Gigabyte GA-970A-DS3
OS: Windows 8