I've had the same issue with two XFX 7970. Both only go about 370 kH/s no matter what I try. I have a gigabyte 7970 in another computer and it runs fine stock at about 680 kH/s. I've yet to find any fixes online  besides tweaking them till they work. Let me know if you get yours fixed (it'd be nice to double my hash rate on the machine  )