It isn't the brushes, it probably isn't the speed resistor, it is a poorly designed motor.
The terminal lug, for the field wire that returns to ground, is attached to the middle of one of the bolts that holds on the motor's end bells. There is a nut on that bolt that squeezes the lug against a piece of plastic. The plastic gets hot and moves out of the way, and the ground connection goes open circuit.
To fix the problem, you need to add another nut to the bolt, and squeeze the ground lug between the two nuts... Your problem, if you are feeling handy, is to get inside of the motor and make that simple fix.
The idiots that designed the motor made a couple of serious mistakes that make fixing the problem quite challenging:
First, when they pressed the motor shaft into the armature, their press expanded the shaft's end slightly. It will jam in the oillite sleeve bearing when you try to take the end cap off. If you feel that happening, you must carefully file down the shaft end (around the circumference) so that it moves freely through the bearing... a fine mill file will do the trick. Do NOT attempt to use sandpaper of any kind!
Second, the bolt that has the field's ground terminal connection also holds the field laminations together. The other bolt does not. That will make the laminations cock on an angle, and jam the end caps from moving. They should move easily. [When you put the motor back together, be sure to add a nut so that both sides of the field laminations are squeezed equally, and that will make any future repairs easier.]
When you take this motor apart, it is crucial that you do not allow the motor shaft, and armature to pull out of the laminations. If you allow this, you will destroy the brushes!!! It is crucial that you do not press on the motor shaft with more than just finger pressure, no hammers, no presses. If you do not follow this rule, you will rip the commutator from the wires on the armature, and your motor will be ruined. DON"T MAKE THIS MISTAKE!!
If the laminations on the side with the loose bolt are jamming the end bell, slide that bolt most of the way out, and stick a punch through the front bell's bolt hole, and press the laminations towards the back of the motor. The bell should come off easily while you are pressing on the laminations... Remember, don't let the shaft pull out with the front end bell!
There is no need to disturb the rear end bell, bearing, brushes, commutator, or armature. Leave them be.
Look for a little capacitor, and a wire with a terminal lug that attaches to one of the motor end bell bolts. That is where your problem lies. You need to remove the nut, and move that bolt part way through the rear of the motor so that you can slide an extra nut under the lug. You want the lug to be squeezed between two nuts.
After you have made the repair to the lug, add a nut to the other motor end bell bolt, so that the field laminations are squeezed by nuts from both sides. You will thank yourself later if you need to get into the motor again for maintenance.
Before reassembling, apply a few drops of oil into the felt around the sleeve bearing.
-Chuck