Hmm I have a 7970 and multiple 6950's running. Could you give what configs you are using?
I'm using GUI Miner Scypt litecoin edit
using CG Miner as my miner
I pick the default CG Miner 7970
Thread Concurrency 8192
Vectors 1
GPU Threads 1
Worksize 256
Intensity 18
At this setting it runs fine, 600-650 hashes BUT NO SHARES ACCEPTED
Now same thing,
CG Miner 7970
Thread Concurrency 8192
Vectors 1
GPU Threads 2
Worksize 256
Intensity 18
Only difference is changing GPU thread to 2.
NOW it works shares are accepted, BUT it works at 200-400 hashes basically a HUGE drop in performance.
The other card meanwhile, the 6950 also in the same computer, is running perfectly with no other tweaks, stock completely, no overclocking or other modifications.