I have solved the mystery of the twitching shoulder. I didn’t cannibalize the old mixer as I’d planned. Instead, I found a reduction gear that meshed with the sprockets I was already using, which made things quite a bit simpler. It also appears the set screws I had been using were too big, and their occasional collisions might just possibly have been the cause of the problem. Both elbow and shoulder joints now move without a hitch.
The next step will be to construct a computer interface to the stepper motor driver board, as I’d prefer this robot to be controlled directly by computer rather than by the BASIC Stamp. I suppose a parallel port interface would work, but since I seem compelled to do everything the hard way, I expect I’ll be building an actual expansion card.