Just flashed my cards with http://www.techpowerup.com/vgabios/126396/HIS.HD7970.3072.120809.html this bios and i'm sitting on 740kh/s stable!

drive 13.5 i kept getting unstable clocks on gigabyte and then flashed about 10 bios's until i found this HIS one.

If this helped you and you want to donate because this took me about 3 days of testing then my address is the following.

LUsybMyNfcfASBAdVvyAmGr1Qqsc7xukUy