I have a 1990 Johnson 40hp VRO outboard on a 16' Jon Boat. The motor starts and runs excellent except for one problem. About every forth or fifth trip out my solonenoid fails and has to be replaced. Seems to usually happen when I have been running the boat for a good while, then I shut it off, then when I go to restart it, nothing. All connections to the battery are tight and clean. This has happened today for the 3rd time in the past 2 months. Each time a new solonoid ($20) fixes the problem, until it inevitably happens again. Any ideas?