It may be the springs on the shift shaft, the springs return the shift drum between each shift, if its beat it pops out. The shift shaft is behind the cover where the shift linkage goes, under there are 2 larger springs, and one small one. usually the regular looking spring snaps,(mine snapped i know) or the other one is just worn. Its not a hard job, and the springs are only a few bucks. change both while your in there.