I have a 1995 Mercury Tracker 90 hp outboard. Was fishing all day no problems motor was running fine. Returning to the boat ramp at very slow speed and motor cut off. Would not start. Starter would run but the motor would not start. Checked all connections a week later and it started. Ran for about 30 minutes at all speeds. Returning to ramp and the motor cut off again. Would not start. What could be the problem?