hi mate, ive had the same problem before with this amp. i bought it new aout 6 years ago and bought another new one about 1 year ago, new, for £70!
on the first amp there was a problem with the surround sound aspect, it kept overloading when turned up even a fraction, it would tick 4 times and power off. Eventually the surround sound stopped working all together and whenever selected made a VERY VERY loud hissing/crashing noise through all my speakers, blowing one of them, SO BE VERY CAREFULL.
when that happened i assumed was broken so bought another one, and had same overload trouble when i got a new speaker package.
Im pretty dam sure it was the ohmage ratings being different on 2 of the surround sound speakers that srewed the 1st one, so id suggest you check theyre all ok, as well as the speaker watt-age.
what are the specs on all your speakers (ohms and watts RMS)??
do you listen to LOUD music through the 5.1 option??
hope this helps, if all else fails buy a new one mate, theyre so cheap. go into panasonic shops and see if they have any left.