From what you describe of the motor starting to turn, then stoping, it sounds like a section of the armature has a broken wire. The armature will spin for 2/3 or 1/3 of a rotation then stop. Check on the wire from the armature section's winding being broken at the commutator plate. Or the solder joint being faulty. If you have a multimeter, check continuity from one armature section to the next. If no continuity, there's a break somewhere. A simple break or bad solder joint can be fixed, but if the break is too close to the winding, you may have to source another armature.