I just now got back to working on this. There was surprisingly little dirt/dust on or near the stepper motor, but everything was quite dry. I pushed the rubber boot on the stepper motor up, sprayed it with silicone lubricant, then daubed some silicone dielectric grease on the shaft/armature. Put the rubber boot back in place and squirted the remaining linkage with silicone.
This seems to have done the trick. I've done the start/stop routine about a dozen times and can see the stepper motor operating now. The bike starts immediately and idles every time.
Now if I could just coax my balky DealerTool software into working for more than a minute at a time, a throttle balance is next. Thanks for the advice, all.