Use your DVM in continuity mode across the power lines, turn the shaft really slowly, a degree or two at the time. The reading should flick as the brushes go from segment to segment, but should go back to some low reading. It could help if you cut out a disc of cardboard and stick it on the shaft and mark it as you go, else you are likely to get lost. If there appears to be areas where the motor is open circuit for any thing more then one degree, remove any covers and inspect the commutator (end of motor with the brushes with copper segments), make sure that the commutator is evenly brown all over. If there are some segments that are black, this indicates sparking , due to excessive current which could mean a short circuit coil on the armature. While you are there make sure that both carbon brushes are the same length and are making a good sliding contact in there holder.
Frank