I also had the same issue with my Sapphire 5850. Anyway rectified with 9.12 hotfix+disable ati high definition audio driver+ changing the default PCIE clock to 101 mhz in bios...Try your luck.
I flashed both my 5870's to the latest ati bios..
did a fresh intall of win 7
installed 9.12
and then installed the 9.12 hot fix on top.
No issues so far ..