McIntosh C22 Unsolved Problem - for techs/smart guys/EEs

Hello, I have a classic Mac C22 that the right channel goes out.  It has been to several techs over the years and never gets resolved (in their defense they have trouble replicating it).  I have done enough control tests to know that: it is not cables or the associated amplifier or speakers, balance control, etc.  I am wondering if someone can suggest what my problem might be based on some of the tell tale signs.  All the caps have been replaced by Audio Classics and it has had a clean bill of health a few times.  Here are some symptoms and clues:
1) The right channel "goes out" (down in level from left channel in varying degrees - about half volume all the way to nearly inaudible, but you can always hear at least a faint signal, and it can be "scratchy").
2) It goes out (or attenuates) either before the system is turned on or while it is playing.
3) If I turn the whole system off, the right channel (still being fed from power remaining in the tube amp) snaps back up to volume comparable with the left channel
4) When returned from servicing, it may be fine for up to 6 months before symptoms appear again.
5) Symptoms may go away for a short time but quickly come back
6) Unplugging the unit (and detaching and reattaching all connections) has on occasion returned the unit to normal functionality. 
7) The unit has worked (and not worked) without changing out any tubes, and it has also exhibited this condition with two completely different sets of tubes.
8) On a couple occasions (but it rarely works) moving the volume knob back and forth quickly a couple times seems to make the right channel pop back in to normal volume.

I know it's hard without being able to examine the unit but I thought it was worth a try - perhaps someone knows something.  Thanks in advance!
Does the C22 have a three prong power cord? What I mean is that , this could be a ground problem. I kept thinking it was a piece of my tube gear as well..(it wasn't)  Are you using multiple components plugged into a few different wall outlets?? If so, combine them down to one, using a cheap power strip that is grounded (three prong). (I had a very similar noisy sound with channel spikes) Good luck.

From what you describe it sounds like the trouble is with the volume control !
Sounds like a flaky tape monitor switch.  Next time it goes out, switch the monitor in and out a few times, see if that wakes it.

I've also seen an intermittent tube socket do this, but on that unit, the pins were visibly discolored and didn't grip the pins well either.
Definitely a bad connection somewhere. I have seen similar symptoms with cracked circuit board traces, cold solder, bad switches and dirty volume pots. If the Mac has circuit boards (not hard wired/discreet) my money would go on a cracked trace or solder connection - they are sometimes nearly impossible to see.  The techs have probably checked out the rest. It doesn’t sound heat related since it fails from a cold start.

Thanks all, I'll try the tape switch.  These units never had 3 prongs, only two, made in 1960...