I have the "765" n scale Berkshire. It was running perfectly until a week ago. Everything works well i.e. sound. It functions well forward and reverse, but it stops from time to time. I've cleaned the wheels, but no luck. I'm not an expert on this, but I think it might be the motor having a "dead" spot. Any thoughts?
Break in the motor brushes. Running the engines forward for twenty minutes at three different settings, then backwards for twenty minutes at three different settings this will insure positive brush contact for good low speed motor control.
Of course you should raise the engine up on blocks off of the rails to do this break in, allowing the wheels to spin freely. Unless you want to watch the engine circling for the better part of an hour while you fiddle with it.