The P2K trucks were loaded up with a low quality grease. It gets
everywhere - such as on the backs of the wheels where it interferes
with the power pickup. Eventually it gets hard and will actually
cause the drive to slow down/not work right.
The fix is to take the shell and bottom cover off of the truck and
flood the truck with isopropyl until -all- of the old grease is gone.
You can see it and it will be a dark yellow if it has solidified and a
less dark yellow before that. Relube with Labelle plastic compatible
and conductive oil (104?) and you will be surprised how well the
loco runs after removing all of the factory grease.
This problem is present in every P2K loco I've ever opened up.
And it doesn't make any difference if the loco has been run
frequently or not since it was built. In fact the worst ones are
usually the ones that have never been run and are just now
being put into service.