I was wondering if anyone had any suggestions to this problem. I have torn the transmission out a few times now and replaced the shifter drum twice and shifter forks once. After installing the new drum it shifts like a dream, then after about 75-100 miles it starts downshifting hard. It then really becomes difficult to shift in general. The shifter drum begins to burr in the channels and the shifter forks then cannot slide on it. Has anyone run into this before and what should I look at? I was considering tearing it apart again, replacing bearings, and checking tolerances. If you have something specific to look for that would help. Thanks.
Sounds like a problem in the output shaft bearings or the primary shaft bearings. Replace them next time its apart and look for funky wear patterns on the gearsets indicating this. Use quality synthetic oil. Hope that helps.