I have two that have not been remotored. One came in a black box, the other in a blue box.
Try these cv settings. On both of them, set cv 2 to 0 and 212 to 255.
Black box engines: cv 209 - 200, cv 210 - 10, cv 116 - around 200, tweak to suit.
Blue box engines: Cv 209 - 255, cv 210 - 1, Cv 116 - around 255, tweak to suit.
I found that operation smoothed out and that they can slow to a crawl. Plus, the pulling power increases exponentially.
Try these settings out out and report back.
What DCC system is used and if you know what is the voltage at the track (load or no load)?
Any DCC system can be used. I have NCE procab wireless. Program on the main yo you can monitor it as you go.