Mmaakk, I want to start be saying thank you for you efforts and contributions to TPU
I have the answer to your problem. In Rivatuner, you must un-link the shaders from the core on the o/c'ing window. You want to take your shaders into the 1800/1900+ range and leave the core in the low 700's. Memory is not as important, but a small boost won't hurt(850'ish). Fiddle until you find your highest stable overclocks. Good luck and watch your PPD climb.