This is what can happen when you have Crossfire enabled without a proper driver profile present. It could be missing or broken. This includes dual GPU cards like the 5970 that use Crossfire tech.
Yes, you can turn off Crossfire and get single GPU performance. Also in that title Crossfire is not needed for decent performance. However if that was a title where you needed two GPUs for decent FPS with maxed settings you'd be out of luck, or have to turn settings down. I'm not sure if the option to switch off Crossfire exists on the 5970 in current drivers. You use to be able to basically accomplish that by unchecking Cat AI in the CCC.
I've used single GPU, SLi and Crossfire across a few generations. As far as reliability, stability and solid support, it goes; single GPU, SLi, then Crossfire.