First make sure the rollers on the trucks are clean. Take a fine Dremel wie wheel and give them a cleaning. //also the tender wheels.
Next remove the plastic tender shell and . inside you will see the relay. on the bottom of thermally there is a flipper contact. Take a fine emery board and clean the upper and lower connects. gently does it.
Then locate the armature shafts in the whistle itself. A drop of oil from a needle oiler at each bearing pointThen put on track and see if it is better.
lousy occasionally - maybe you track needs cleaning - the track does build up a carbon/grease deposit. Use a green fiber kitchen pad - not BRILLO a fiber pad and some alcohol - the denatured kind not the drinking kind. and clean your tracks. use a little bit.
Then when all is done and said if it works good. If not then maybe the contacts on the transformer need cleaning. Should work with what i said.
let me know repairinfo at juno.com.