Had a pinball party and tournament at my place, and lo' and behold the local master technician stopped by and volunteered to chase the gremlin out of the machine.
It was switch 37 after all: the wires were apparently backwards. Fuuuuuuuuuuuuuuuuuuck!
We had replaced the switch, but I just kept the wires the same way they were from the factory. Which was wrong.
Never assume they got it 100% on the assembly line, right?
But that was it, problem disappeared, game is 100%.
I have someone in line ready to borrow it, and they got to spend time with it on Sunday and I think they might be falling in love with Corvette. We'll see!