Marantz 7200: strange distortions

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My Marantz 7200 I use for a home cinema theater (it has nice surround sound processor) started to distort as if something is heavily clipping in all channels. I opened it on the bench, looked inside, but did not find any bad solder or something similar. When I connected a signal generator and a dummy load it worked fine. I kicked all boards by a screwdriver, no result.
I put it back and it worked fine for a while, then started to distort again suddenly.
What is strange, internal FM radio sounds nice.
Any clue?
I have no schematics, unfortunately. It has lot of small PCBs soldered vertically and fastened to the rear panel, so it is very hard to troubleshoot...
 
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