Did you clean and sand the motor commutator plate? brush tubes? wipe brushes & check for flatness? springs arent weak? All connections are strong?
With a stiff paint brush or lightly dragged pin/toothpick make sure no brush dust or gunk is in the gap between plates. Lightly because there is wire in the gap sometimes, you dont want to break, score it, etc.. Debris means the electricity travels there, bypassing that sections wire loops that make up that sections electromagnet. (there are basically three moving magnets turned on and off by position)
(pressure on connections is important, including spring pressure on brushes).
Disassemble the brush cover, take an ohm meter to the shaft and each armature plate segment; there should be no contact.
Take a reading between plates, there should be no large varience.
Take a reading from stationary field core plates to the winding ends; no contact.
Reading from coil end to end should make contact like any other wire from end to end.
Also as a last resort sort of thnig because you really dont want to upset it; make sure the armature plate can't twist on the shaft at all. Ive had two get loose, cause iissues, and just swaped in new ones; but I don't know if thats a special armature or swapable with a type 1 or something.