Everyone, thank you so much for the help! I've always thought my troubleshooting skills were decent, but this one has me wondering. Your input is really helpful, I appreciate all of it.
Also, check for any connection between the wheels on each Axle. There should be none. Then, fully assembled, check for connection between all wheels on each side.
I checked resistance between the wheels when fully assembled. There was the expected connections between wheels on the same side. AND there was about 200K Ohm from one side to the other. Removed the wipers/cover and checked again but had no connection between right/left. So something about the assembly was causing a high resistance short. I cut some small pieces of shrink wrap and laid them over the axels and assembled again, this solved the resistance problem. However, the loco won't run with those pieces in place. So I used the clear nail polish again to coat the portion of the wipers that is inside the assembly (the blue box in the image below). This resolved the connection between the wheels. Now, I read under 5 Ohms between wheels on the same side, but OPEN from any of the left/right combinations.
I had a lot of hope after this, really. The shorting between wheel sets seemed to be the type of issue that would cause intermittent shorting which results in jerky movement. But no. Operation is still the same.
Not sure: Have you put power to the black and red leads while it's not on the track ? I.E. If, while on its side it still baulks/stutters would eliminate wheel pickup as culprit ...leaving wipers and motor...and I'm thinking at this point it is going to turn out to be a motor problem...
I have not done this, and now that you say it, I'm not sure why I haven't. Seems to be a logical
first step, but haven't done this yet. I'm working from home today, maybe I can try this at lunch time...
Remove the connecting rods then try again to see if there is any mechanical binding there. That would rule it out.
This is what I want to do next. Everything looks electrical, but at this point, I have no clue.
To remove the wheels takes some effort on this loco. Will update when I get this next.