Lurching MTH PS3 611

Mechanical binding possible, can be a slew of items from gear box, flywheel rubbing, traction tires, linkage etc....  G

I'm not saying this is the problem but a known thing on that run and a few recent large MTH steam engines produced was that in the tender, the wiring harness that plugs into the PS3 tender board was zip tied so tight that they were prone to unplugging connectors. The PS3 tender board is mounted using the rectifier and so the board just hanging mid air can flex and with any rough handling the board flexes and can unplug or partially unplug the connector. Mostly that problem was related to the lights and potentiometer volume and smoke controls but it's one of those "fix it" items to check on this specific engine that I learned about after seeing no less than 3 of these.

Thanks for the posts.
I have already had the tender open on my 765 to fit the sounds that weren’t. I can check that on this 611 without a problem. Next I’ll move on to the traction tires and progress from there. It’s out of warranty, so I’m not adverse to taking off the shell, and if I can’t easily find it, I’ll get it to a repair center.

Check the brake shoes for rubbing on the traction tire. I always take off the brake shoes when weathering and do them separately. When I reassembled engine that previously ran perfect. It  ran much as you described. Loosen the screw and wedge something between the shoe and tire and re tighten.