Is this the President's Choice train set?
Further to Wayne's comment, with cheap motors with few poles you get an effect called "cogging" at very slow speeds as the motor zips one pole to the magnet then holds a bit and then zips another pole to the magnet. (very simplified explanation). This wold give many jerks per wheel revolution.
Do your jerks match the wheel revolutions (or twice a revolution)? That would suggest mechanical binding. If the loco keeps going deoite the jerks, it's probably not dirty wheel-rail contact.