I have a problem I cannot explain with my dual HD5870 setup. I recently replaced my crossfire bridge (catalyst gave a message that my bridge is probably broken). Before replacing the crossfire bridge I had grey patterns all over my screen while running a game. So I bought a new crossfire bridge.
But now when I have crossfire enabled and i am playing a game like Counter-Strike: Global Offensive i get around 70 fps.
When I have crossfire disabled I get 300+ fps.