Why are you using Crossfire? It slows down mining. If you are using it for gaming, lowering memory speed doesn't help that either. o.o
My Afterburner freezes too when I do this, I have a 6990 + 6870.
I'm using Crossfire because the computer does more than mine for bitcoins... I want to lower memory speed to reduce heat and save energy, nothing to do with hashing performance.