I had a thread going about this issue as well.
It is here and may have some useful info -
Lets Solve the unreliable 7970 issues.... (share your observations/input) -
https://bitcointalk.org/index.php?topic=174036.0Frankly the most frustrating aspects of this are two things, inability to duplicate results across different brands/versions of 7970s, and absolute non-linear results. One card 1022/1244 might get 650k, 1022/1450 get 400k, and 1048/1720 get 720k.
They are the fastest cards but something tells me there is some sort of bug we have not found - for example most other cards show little performance variation and there are formulaic settings that can be plugged into any card to get close to max result.
Not so for 7970. I still have 2 that are fubar. I don't think I will ever get either one above 600k 24/7.
It's like a gearbox, with the core, memory speed, thread concurrency, gpu threads, and who knows what else, all as separate gears. If you don't have quite the ought ratio on any of the moing pieces, your hash rate suffers.