I've just recently noticed it, but for all I know, it may have been happening for a while. It's not something I've noticed, but I'm afraid it's affecting game performance. (Super paranoia thanks for Battlefield 3 :])
Catalyst Control Center used to let me turn Crossfire on and off, but now I can't find that option.
The laptop/mobile HD48XXM models use less power than the desktop versions and they aren't directly comparable. Regardless, heat is the issue if idling at 76C. Time for an RMA/service. Sorry to be the bearer of bad news. Good luck!