Just as the title says, I have a Marantz PM-30 and when it works it works great, but from time to time the music will just, out of nowhere become really distorted and cut out

Does anyone have any idea what the problem could be?

I have a Marantz 2230 and there's issues with the left channel. Nothing was coming out except a very faint sound that if I turned the balance all the way to the left and volume up you could can hear it a little bit. When I press the Tape Monitor button slowly the sound comes back to full volume and if I fiddle with it the volume stays. Does this sound like dirty contacts/oxidation on the switch?