Yeah, I had the exact same thing happening to me with my 5970 in true full screen.
I think it must be xfire issues. They simply do not have it working at all well.
I eventually just accepted it and have since changed to a GTX580 3 gig card and the game runs smooth consistently now (although I still get the stutters like everyone else).
Not really a smart solution though, buying a new high end card when the 5970 is a fine card in its own right...but that's what I did.
cheers
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