I'm also fighting with 5970, those GPU are not easy... Can you tell me which Catalyst and AMD-APP-SDK versions are you using and which cgminer options?

For reduce the VCC you only need to add the next option to the cgminer.conf file

    "gpu-vddc": "0.95"

But in my tests less than 0.97 gives very low hashrate... however maybe I messed it up in some other point.