N Scale Northern 4-8-4 steam locomotive and tender runs jerky and not smooth

[Chewey]

Problem locomotive at slow speeds runs jerky and not smooth.

This is the second N scale Northern 4-8-4 steam locomotive, the first I sent back to Bachmann and they replaced it with a new locomotive and some light oil. The second one, I have applied the oil to the external linkage which has helped.

Is this an inherent problem with N scale steam locomotives or is there anything else I could try?