So yeah, just before putting away the train set for a bit, I just thought, because Gordon and James have the same motor, run James on Gordon's motor and see if it is actually the motor. And it runs well. I did clear out a bit of gunk from the gears but didn't do a full strip down of the motor. But it doesn't explain that surely. So I'm guessing I need to clean or replace something on Gordon's chassis/main body, any advice is appreciated.