I had a '95 Stratos with a '95 Evenrude 150. while fishing once I had a similar problem. Somehow a stick that was floating in the lake had gotten stuck between my motor and the boat, and somehow that triggered some type of safety switch. Once I removed the stick, it worked perfectly.
Don't know if that is your problem or not, but worth a look. Also, do you have more than one switch on the boat? For instance, my boat has a total of 4 places to trim or tilt. Could one of the switches be burned out?
I'm no mechanic, but I can sure tear things up.
Good luck,
Danny