'90 50 hp Merc sometimes no power to start

[Mr.Outdoors]

I have a 1990 50 hp Merc that has worked great for years. Yesterday I went out and it started right up and I went to the middle of the lake. When I went to restart it again nothing. I checked to make sure it was not in gear and it was not. I then checked the power trim switch and nothing. Its like the battery was disconnected. I checked and the cables were tight and the battery was good. It did start about 5 min later, but did the same thing about 30 later. When I got home last night I checked and all the connections on the motor appear tight. I took off the gear controller and opened it up and I could not see any loose wires. Does anyone have any advise on what to check next? I want to use the boat this weekend but I am concerned about it not starting.