I have had motor slow down happen on several Bachmann locomotives, with the small can motor. One responded to cleaning with CRC electronic cleaner, injected with a syringe and then a "high speed blow out". As has been suggested.
However, a recent purchase was made worse by doing this. I didn't want to do it, but I pressed the flywheels off and opened the motor up. No brush/oil deposits were found, but I did have a winding with one coil pinched under the thrust washer on the motor shaft, opposite the brush end. It was close enough to the motor shaft, that under load it would "ground" to the the motor shaft. The armmature was tested with a VOM for grounds, shorts and open coils, but tested OK. Why? No pressure on the thrust bearing when not installed. The engine didn't fail in one direction, which makes sense, because the armmature moves against it's thrust bearings depending on the direction of rotation.

If you have a ammeter on your layout (if running DC) you'll see the currect draw go up when the loco slows down, mine went from 0.2 A to 0.4 A.
Once put back together, the motor runs very good now and draws 0.1 A. I think the motor winding machines are a bit "sloppy" and the QC maybe lacking too. If I were you, I would contact Bachmann and get a new motor or loco. Charley.