I'll follow up on my own post. Joe Gore on the Yahoo list suggested that the motor shaft or leadscrew end might be slipping in the coupling, even though the clamp screws are tight (to the point of my having bent a hex wrench in checking their tightness...) I scribed lines on the coupling and leadscrew locknut and put a Magic Marker dot on the motor shaft, all in a line. Sure enough, after 500 of my up-down cycles the motor shaft has rotated maybe 10 degrees relative to the coupling.
My next task is to block up the head, remove the motor and clean and degrease it and the coupling and see if that is enough to resolve the slipping. If the inside of the coupling is as roughly machined as the outside, I might end up making or buying a new one. And/or I could add a setscrew to the coupling to engage the motor shaft flat that I can just see peeking over the end of the coupling.