I think you should try updating your CCC and also make sure you are flashing to the reference bios. Then after that make small tweaks I can only seem to get 640kh/s with my 6x7970s but with just one I can get 690. Also my settings are 910 core 1370 mem, I spent hours trying to get it to go higher going up but i took it down a bit and got a 40kh/ sjump