In addition to what Joe said, did you check the wear of the motor brushes themselves. If you can see half, or more, of the brush holder above the brushes they should be replaced.
Replacements are available from Olsen's Toy Trains (PN: 622-121) for $1.00/pr+S&H. Or you can get brushes with a copper 'wick' imbedded from Lionel (PN: 6108507150) for $1.20/ea+S&H. The wick gets soldered to the brush holder with the e-unit wire so connectivity isn't completely dependent on the brush spring.
Len
Replacements are available from Olsen's Toy Trains (PN: 622-121) for $1.00/pr+S&H. Or you can get brushes with a copper 'wick' imbedded from Lionel (PN: 6108507150) for $1.20/ea+S&H. The wick gets soldered to the brush holder with the e-unit wire so connectivity isn't completely dependent on the brush spring.
Len