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  All the caps have been replaced by Audio Classics and it has had a clean bill of health a few times.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!
Thanks all, I'll try the tape switch.  These units never had 3 prongs, only two, made in 1960...
I have a C-20 with a similar problem, except my L-R balance is always terrible. (Instead of pointing my balance control straight up toward the 12:00 position, I have to point it to about 2:00 - 2:30).  I have also had it in a few times and have had the caps and a few other parts upgraded.  But the L-R balance is still way off.
I also suspect it is the volume control and have made arrangements to have a new, upgraded volume control installed.  I don't know if they make upgraded volume controls for the C-22 but I could ask while my C-20 is in for the service.
I'm scheduled to have the service work done later this month or early October (depending on how fast work gets done on current projects).
If you like, I can keep you posted of my results, and inquire about your C-22 as well.
PS If you decide it's the volume control and live near Chicago, I can tell you where NOT to go. 

I would say to replace the potentiometers in the volume and balance, like toolbox indicated. These potentiometers are basically variable resistors that electrically control the gain of the signal going through the circuit. They can go bad and can also cause intermittent issues, like you have experienced.  The bass/treble potentiometers could possibly affect the circuit too, but I would do the volume/balance first.
What about tapping the unit? Does that ever help? If not, try tapping, or flicking the tape, volume and balance controls.

Follow up - so I shipped off the unit to Audio Classics as a last ditch effort (I live on the opposite Coast) knowing if anyone would know, they would.  I got it back a few weeks ago, and it has been working withhout issue.  As always, they did a great job; I felt like I was getting a new piece of gear in the mail.  OK, so what they said was that in the last five years or so they have found a measurable phenomenon with the RCA jacks in a bi-metal corrosion issue.  If I get this right, the jacks were made with a combination of nickel, aluminum, and perhaps some other metals.  The engineers presumably never anticipated this stuff being used  50-60 year later.  With extensive use, friction, etc., and corrosion the overall wear to the metal(s) changes the resistance.  They said that the symptoms I were having were typical of this problem.  Not sure I got that correct, but bottom line they changed all my RCA jacks with new ones (as well as some other stuff to bring the unit up to specs) and it works great !  Thanks to Audio Classics.