My noob 2 cents
My 2-6-0 ran great (non spectrum Bachmann), did the break in, after a few weeks sounds like similar issue, "erratic" running, chugging, stalling. Then I read posts about cleaning and lubing from the veterans here. I read the sheet that came with it from Bachmann explaining contaminants in the pickups and brushes etc etc can cause this "erratic" running. Got the Bachman plastic safe conductive lube stuff. Blew it out with some RC car plastic safe electric motor cleaner (not recomended method lol but a lot of black gunk came out of the motor etc either from manufacturing remnants, paints, or crud from running under the xmas tree as a Berkshire lol) then I lubed it and now it runs like a swiss watch again. Hard part is getting the shell off/on, still not sure how to do this properly? I got frustrated/scared and just left it on this time. (scary) Good luck, let us know how it works out.