You probably have worn brushes - one wears first and begins to lose contact with the commutator. Remove the brush holder assembly and look at the ends of the brushes - if one is a darker colour than the other, that's the trouble. Replace the entire brush holder assembly with internal regulator. The Taiwan made ones are good quality, and far cheaper than the Bosch ones, which are actually not very good. Simple job. Unless someone installed a battery backwards, the diodes will be fine.