o-4-0 side tank porter runs erratic

[on30francisco]

My Large Scale 0-4-0 side tank porter that I purchased several months ago runs very erratic and seems to be getting worse. It must be manually nudged to get it started and jerks while running down the track as if it isn't making contact with the rails or has a bind in the mechanism. The problem isn't my track as my other locos run well. I have ran the loco thinking it would improve as it got broken in but this problem seems to be getting worse. Has anybody else experienced these problems and if so, how did you correct them? I am operating indoors in a controlled environment so the weather is not to blame.

[Curmudgeon]

Which Porter?
The new 1:20 one, or the older 1:something one?
If the former, to provide chuff contacts, the pivoting front axle for 3-point suspension went away, making them rigid chassis.
Does the headlight stay on or does it go off when it stops?
Have you opened it up and lubed it?
Contact lube on the axle wipers?
Would help to know those things prior to continuing.