Just a guess, but replace the rear speaker cables and see if it still happens- it could be a short in the speaker wires. My receiver was clicking in/out like that and it was a bad splice in the wire: the +/- wires were crossing. (My fault on the splicing.)

I also had the same problem when I hung my rear speaker wires with thumbtacks. Needless to say I've gotten smarter about it now!

-Steve