Hi Alex,
You appear to be experiencing some of the same problems that hindered my enjoyment of British N-Gauge and speeded up my change to its European equivalent, which, at the time tended to be better engineered.
It must be nearly 20 years since I last dabbled with British N-Gauge but from what I can remember poor running was often due to incorrectly adjusted wheel pick-ups. If they were too slack electrical contact was sporadic resulting in jerky operation and if they were too tight the extra friction hindered slow running. This wasn’t always the result of wear and tear as I found brand new locos straight out of the box often needed tweaking.
When trying to track down the causes of poor running it often helps if you can find some way of disconnecting the motor drive from the chassis. This enables you to “free-wheel” the chassis whilst you feel and listen for tight spots caused by the pickups or possibly the connecting rods.
Another common problem with older Grafar steam locos was uneven meshing of the motor worm and the wheel axle gear. There was often excessive float in the motor armature and this usually meant that the loco ran better in one direction than the other. I found that it could be improved by adding shims or slightly adjusting the motor position until the optimum position was found which gave smooth running in both directions.
I’m sure that with a little patience and some trial and error experimentation you can become your very own serviceman.
