Windows 7, changed many GFX drivers. I have 3 7970 card and using the riser with power supply on it . Why is my 3rd card only hash about 360KHS while other cards hash 700Khs. If I switch the card order its always the 3rd card, so hardware does not have problem.
Its your settings, try playing around with your GPU-thread 1 and 2 and the threads count... leave voltage, engine and memory clock alone until you get the best threads...