Try these from this site:
http://forums.hunter.sailboatowners.com/showthread.php?p=720295&highlight=transmission linkage
Could be the linkage or the transmission itself.. If you search on “adjust linkage” too, ya may find some good stuff.. Someone had posted the procedure to adjust the internal transmission linkage.. but don’t try that until ya loosen the remote linkage and see if the tranny shifts fine when done manually.. Be sure to keep human parts out of the rotating things .. beware of shirtsleeves and the like when checking this.. good luck, Rich