Replace the solenoid. There is a round copper disc inside and that is what makes contact to start the motor. Each time you use it, it rotates a little bit. Over the years that disc gets burn't and when it gets bad enough it no longer makes contact. Until it rotates past the burnt spot. That's why sometimes it works and others it doesn't.

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Fair Winds and Following Seas

Bill H. PTC USN Ret
Chesapeake, Va