Advice;
- Dump GUIMiner and learn to love CGMiner directly
- Install either Sapphire Trix or MSI Afterburner
- Overclock your cards' GPUs to at least 1130MHz. Downclock memory to 600MHz to save energy and generate less heat
Should yield you a stable ~685MHs hashing on a 7970.
So far, this is the best advice on this thread. CGMiner is awesome. I'm getting these same numbers stable on my 7970 rig.