Does it look like that in Unigine Heaven? If it does, possibly a wrong/corrupted video card BIOS! Especially if the temps are OK. I had a GeForce GTX 660 Ti that was given to me and it got flashed with another vendor's video BIOS and it would look like that in Unigine Heaven. Then I decided to monitor the video card frequencies and the GPU frequency was bobbing up and down and utilities were showing false TDP throttling! Nvidia Inspector showed the my eVGA GeForce GTX 660 Ti had an Asus BIOS! So I flashed it with an eVGA BIOS that I found at TechPowerUp and it was normal again!