I have a 6990 and Debian unstable, which includes fglrx 11.9.1. I have tried AMD SDK 2.4 and 2.5. Either way, DiabloMiner uses about 90% of a CPU core when mining on one GPU, and about 180% (nearly 2 CPU cores) when mining on 2 GPUs. It also causes lag when trying to play games on the primary GPU while mining on the second, which worked fine on fglrx 11.4. I understand this is a driver bug, but how can I fix it? Which driver can I install and how? There's no fglrx driver at all in Debian testing (presumably due to bugs in 11.9). Debian stable has no support for AMDOverdriveCtrl so I couldn't underclock my RAM. I tried to downgrade my deb packages, but then got dependency problems with libmesa and couldn't install anything new.
Would AMD's installer for an intermediate version, like 11.7 work better? Or can we just fix or report the actual bug in the driver?
Never try to use AMD's installer on a distro that provides packages. It usually just causes a mess.
Your only choice is to wait for AMD to fix it, but I suggest you keep trying to report the bug.
Games are probably lagging due to the CPU use bug, just turn DiabloMiner off when gaming.