Jammed shifting system on 2016 Mercury 9.9

[Capt'n Caution]

This motor has jammed 3 times. The gears are in neutral, the starter cord cannot be pulled and the twist grip cannot be moved. When depressing the throttle release button, the twist grip will adjust the throttle, but all else is still locked. If on the test stand, I can rotate the prop with my foot and it un-jams. If on the water, I remove the cover and gently pry up the locking lever on the starting cord reel, then I can start in gear with a low throttle setting. I can then twist the throttle and get into neutral. I took it to the dealer twice, but road vibration unlocked the jam so nothing was visibly wrong. I am concerned about this happening in a dangerous situation on the water. Anyone had a similar experience with the Merc 9.9 or 8 hp motors? Any ideas?