Originally Posted by thestache
Alright thanks for the clarification guys.
I'll use GPU-Z and keep trying. 1300mhz is stable at 1.212v and works fine on a restart but with the with 1.35v applied the same clocks or even the default clocks of 1100mhz artifacts and I get 30-40% GPU usage and terrible performance. Might just be a driver issue and need a clean and reinstall. I'll try that since everything else seems fine.
You might be hitting the artifact temp limit, card often goes up in a mayhem of artifacts around 80c, try backing the clock off to 1200mhz @ 1.28 and work your way from there, leave the mem default for now. Aim for a maxtemp around 78c. Most voltages over 1.25 will require 100% fanspeed or an aggressive fan profile.