Have you tried just the driven axle, under power, with wires soldered directly to the motor? If that's OK, then try running the chassis as an 0-4-2 and then a 2-4-0 to try to identify where the problem might be.
I have had the motor attached to the controller directly & it's fine. There's no binding what so ever on the chassis it's beautifully smooth to the point I can blow it in both directions. I'm going to post in the motor & drive section & post because this is on 2 chassis.