|I do not know crossfire well but I do know that it can be power hungry. Post all of your specs, especially your power supply make and model number so we can see if it is sufficient to handle your complete set up.|
Another thing you can do i the mean time is install HWMonitor, launch it, do non gaming things for a while, then light gaming, then multiPlayer and look at the various temperatures, voltages, and even fan speeds and post back the results (min & max). These may also help find the problem.
You have to be a little bit crazy to keep you from going insane.