I have AMD A8-3870K APU crossfired with HD6670 and the game played smooth with 12.8 amd driver but when i updated driver to 13.1 it started to lag even in main menu and i would get in game min 15fps max 24fps. This was also problem with AC Revelations but somehow it fixed it self. I have tried to uninstall 13.1 driver and install 12.8 and games work fine but windows explorer and other programs are crashing so I install new driver again and windows is fine but games are not.
P.S. I am know running driver version 13.4 and everything is same.