I play it with my 5970 @ 1920x1200 with everything maxed out(16xAF,0xAA) and it runs fine without any FPS drops or stutters(however sometimes when it wants to load a new area,FPS goes down but goes up to normal again),i am using Catalyst 10.3
Do you have the latest drivers installed ?
Disabling Catalyst AI disables CrossFire,i run it with Catalyst AI enabled.
But for some games like PES 2010, runs better with Cat AI disabled,so test some games and if in all of them your performance was worse with Cat AI enabled then there is a problem with the second core but if it worked fine with most games then i think its a driver issue and will be fixed
BTW,is Vsync enabled or disabled ?
Its not possible
Disabling Catalyst AI = Disabling CrossFire
When you disable Catalyst AI,you have disabled CrossFire,(go to a game and check GPU-Z and you will see that the second core is inactive with Cat AI disabled)