A possible cause might be the infiltration of humidity or moisture into the volume level control potentiometers.
I would suggest getting a spray can of electrical contact cleaner from your favorite electronics store, taking the case apart to where you can give any control pots or other switches a few pervasive sprays of the contact cleaner .... Allow to dry and reassemble.
Also, spray the RCA connectors of the audio outputs. ... Look for any switch or anything else that looks like it might move, and give it a spray or two.
This procedure often yields a performance enhancement, but is not too difficult to accomplish.
If the problem still persists after the spray cleaning effort, you should try to find a schematic of the unit, and proceed from there.
Electrical contact cleaner sometimes comes with a silicone compound. I would suggest that you initially try the version that does not have the silicone ..... I would not use the WD-40 product for this purpose.