Sounds like it's overheating or it's a driver issue. They go crazy when they overheat. Have you tried monitoring the temps while you're playing? Also, have you dried removing the drivers, using driver sweeper, and reinstalling the drivers? If you suspect heat, go to Catalyst Control Center, select the graphics tab, click "Enable Manual Fan Control", set the fan speed to "100%" and click "Apply". See if that helps your temps.