I have a 1981 mercruiser 5.7 260 hp with alpha 1 outdrive. This unit shifts smoothly when engine is off and when first started. After it warms up it is difficult to change gears. It acts as though it is binding up. You can get it out of gear but you really have to fight it and many times you end up going past nuetral and into the next gear. The kill switch does work. I tested it and the engine does shut off. It has been suggested that there is a problem in the lower unit and that when the gears heat up they are not meshing properly. Any thoughts/suggestions?