Take all cables off, sand the connectors a little to make the copper shiny, and bolt them back up. Ground cables, red cables that look like battery cables, etc. To and from the battery, to and from the starter, and all grounds.
This is common on boats and watercraft. The connections get corroded over the winter.
Also, grab the copper ends and wiggle them a little against the wire. Sometimes they are rusted away just under the wire insulation, and they will snap off in your hands.
Warning, if you DO find a bad cable, it will be one that connects in the hardest to reach place. :ack: Those ones get the most moisture on them and corrode the most.