A snippet from Tom's in their
Review of Crossfired AMD Radeon 7990s:
Performance wasnt even my top concern when I got my hands on an additional two Radeon HD 7990s. We already know that dual Tahiti-based configurations run into issues with dropping and truncating frames in a number of titles. AMD knows this too. The company has a driver in development intended to achieve better pacing between frames. I previewed it in the Radeon HD 7990 review, and it looks promising. Today is not the day that driver becomes available.
So... we're still waiting for the fix.