It can be a motor problem. The bearings can start to lock up or if the machine has been over oiled the oil gets down
into the brushes and on the commutator. The motor can easily be removed. Then it can be taken apart and degreased
with an electrical contact cleaner and then the commutator and brushes lightly polished with a 600 grit emery paper
to remove oxidation caused by the oil.
This sounds like a big procedure but actually isn't hard to do on these type motors.