Just a shot in the dark, but are the fingers on the reverse unit drum in their correct position and is there any "crud" on the drum? With enough build-up of crud, there may be electrical conductivity between the brass sections. The fingers should be nice and clean with no holes on the contact areas of the fingers (but tha'ts not all that necessary - I've seen "holey" fingers work without shorting out). The fingers should be in the center of their respective brass contact segments. Also make sure also that there is no contact between the fingers and other parts of the motor frame.
Sounds like kind of a fun problem in troubleshooting. At least for me, who isn't trying to diagnose the problem.