Does a 3.8 kernel it work ok if you revert mesa back to
cf5632094ba0c19d570ea47025cf6da75ef8457a?

I think
r600g: rework flusing and synchronization pattern v7
http://cgit.freedesktop.org/mesa/mesa/commit/?id=24b1206ab2dcd506aaac3ef656aebc8bc20cd27a
may be problematic on cayman.