Intriguing! I "fixed" the problem last year by running the pots and switches back and forth for a few minutes. Lately, I've been doing that 10x after every listen. The problem is back, and worsening.
So. There are some downsides. I don't have any electronics expertise. I'm not sure how far I'd have to take things apart -- maybe just the cover to get at the pot, but the A/B speaker switch seems to be enclosed, and it's a problem. The unit has seen a lot of use in 31 years. I could order some DeoxIT, pay the bucks, wait, wait until I have a free Saturday, ... and it might or might not solve the problem. I do wonder what kind of shape the other components are in. Replacing caps sounds like a smart idea. I have a dealer I trust, but he's 2 hours away. So that would blow two Saturday afternoons, plus a chunk of change for the labor. I know, patience is a virtue, right? Or... I could just move on, and make this someone else's project! Just for kicks, how much do you think an NAD dealer would charge for a full cleaning and re-tube?
As always, thanks for the advice!