I'm having the exact same problem with my new 842. Please keep us informed as to what the solution was-thank you, joeldee-berlin
I was excited to receive a new #844 UP 4-8-4 but unfortunately it has a problem I thought I'd share. Bottom line is it's going to Athearn for repair. When run around the layout it would occasionally stop, sometimes staying there and sometimes quickly getting going again. Resets of the decoder had no effect. The stalling seemed to occur at certain spots on the track, not turnouts, and eventually I figured it was when entering / exiting curves. Track was super clean and other locos just don't act up like this nor in these spots (i.e., not a track issue). On inspection I noted the loco plug (to the tender) had the right-most wire showing some bare wire, where the other 5 were fit into the plug brass connectors with the insulation flush. I don't know if the wires just slide into the plug connections or are soldered in. After consulting with David at Athearn we tried pushing the wire in, bending it away from its neighbor, etc. The frequency of the problem might change but the problem always resurfaces. We also swapped my similar Challenger tender to the FEF-3 but the problem was the same; proving the issue is with the FEF-3 loco. Boy, the contacts where the wires go in the plug are close together. Anyway, it goes back tomorrow. I suspect either the wires or contacts are touching each other with the movement involved (a short) and/or there's a wire not properly seated in the plug (a temporary disconnect). I'd bet a new plug with proper wire connection would do the trick.
My 2013 Genesis Challenger and Big Boy have a similar setup but do not exhibit the problem.
Paul
Modeling HO with a transition era UP bent