Try disabling Crossfire. A lot of games do not work well with it enabled, and sometimes you can get negative scaling. The fact that your 5970 is quite old and doesn't get driver updates anymore probably isn't helping. Also keep in mind you may be starved for VRAM, the 5970 might have 2GB of VRAM, but it only has 1GB for each GPU as it is a dual GPU card, and that is below minimum requirements for a lot of modern titles. You're probably going to have to play at 720p low settings if you want a somewhat smooth experience.