Not sure if this helps,
But I use SDK 2.1, and drivers 11.12 with 3 5970s and 1 5870 in a rig. (7 GPUs) I have not gotten any errors at all, or YET.

Make sure you don't install the actual SDK EXE installation, but extract it with WinRAR and only install the SDK MSI file. That got my hash rate up a bit.