check your shift cable to the lower unit

with no load on the prop there is no resistance in lower unit to the shift cable. so you may not see shift interrupter deflect. put it in the water. if interrupter never moves, it is hard as hell to shift out of gear. usually when you need a new cable, the cable drag causes interrupter to take too long to pop back into center (uninterrupted) position and stalls motor. EDIT; I got to bottom of first page and wrote this. then I saw 20 other posts where this was already answered.