As CK says of cgminer, the 7970 requires the memory clock to be no more than -150 than the engine clock.  Per the cgminer readme:

--gpu-memdiff -125
This setting will modify the memory speed whenever the GPU clock speed is
modified by --auto-gpu. In this example, it will set the memory speed to
be 125 Mhz lower than the GPU speed. This is useful for some cards like the
6970 which normally don't allow a bigger clock speed difference. The 6970 is
known to only allow -125, while the 7970 only allows -150.