Despite everyone saying its all about drivers it's not.
I installed 4x 5970 cards on windows 7 (new machine etc) 12.xx drivers and shit didn't start working properly until I put in dummy plugs.
Just sayin...
LET THE FLAMING BEGIN!
Did you install ccc or new drivers at some point? Maybe you didn't erase open.cl and amdocl from your system folders when you downgraded. I know my hash rates were terrible until I did that.