Talked to Steve, consensus is that I need new gearbox/motor assembly, and it's on it's way.
Funny thing is, and I wouldn't normally do this but as long as new parts are on there way...., if I spin the gearbox by hand, meaning that the carousel is attached and I use it for leverage to turn the gearbox, the gearbox actually spins both forwards and backwards. It doesn't sound or feel like there is anything broken inside. And then, as I increase the force/torque on the carousel it seems that the shaft slips within the gearbox. Not the gears skipping a tooth, but the interface between the shaft and the gear it is attached to. Not sure if this is a press fit or not, but it doesn't appear to be a spline.
Whatever it is, there has to be a weak link somewhere. Something's gotta give, either mechanically or electronically. My constructive criticism for SP is that this weak link should be serviceable by replacing a fuse or a resetting a circuit breaker or designing in a pin that could easily be replaced if/when the carousel gets jammed. On the other hand this seems to be an unusual failure, so perhaps it's better to leave well enough alone.
Being an engineer, I may have to open up the gearbox to see if I can identify the failure......