Clean or replace the relays.
I replaced the relays in the TX-904 I had. Then I figured out how to clean them and did that to both an M-501 amp and TX-906 to great effect. The 906 is switched on/off 20 times a week for the past 18 months and those original cleaned relays are working just fine.
This is of course for a unit that has no output in one channel, all sources, and once a volume level is reached the channel will come back. May or may not drop out again while on but will drop out next time the unit is cycled off.
Black relays, top comes off, move the teeth out of the metal tabs carefully, flip the teeth unit back out of the way and then clean the contacts.
I used a thin piece of 600 or higher grit sandpaper and ran it through a couple few times. Blew out the whatever a few times and checked it, buttoned it back up and enjoyed the results of my labor.