I appreciate all the responses. I've worked and inspected the mechanisms as suggested looking for any sign of binding, loose bolts, bent parts, play in the rotating parts, and excessive wear or chips on the gears, and none of those have given me any clues. I'm actually impressed with how little slop there is in the drive mechanisms when I turn the center wheel. I'm 51 years old and this set was a Christmas present to my Dad when he was a child. I know I ran it pretty hard when I was a kid, and I expect he did too long before I was born. It was only set up around Christmas in my lifetime though, so not abused for most of the year.
I'm not very handy with a soldering iron, so I'll go back and work through all the other suggestions a couple of times and probably stare at it for an hour hoping for an "Aha!" moment before I bite the bullet and start monkeying with the wiring.
