'99 Mercury Kicking Out Of Gear

[Coast Ranger]

My '99 Mercury is a 70ELPT. Yesterday we were down the river, beached up for a most pleasant afternoon. When it was time to head home, the motor wouldn't start. When I turned the key to the start position, the gauges all came up and I could hear a humming in the motor. We "played" with the controls, shifting to forward and reverse several times, and then it started. While motoring back to the landing, the motor "stuttered" twice, like it jumped out of gear. Fortunately, it went back in and we made it back with just a little anxiety.

Isn't there a switch or sensor that keeps the motor from starting in gear? That's what I'm guessing could be the problem. I need to confirm that, identify it, and figure out where it is. I would also like to know if I'm on the right track or if there's something else I should be looking at.

Thanks in advance for any help,

Tom