Our Yanmar has a switch and a starter button. I had the exact same problem except that it would sometime just click. Sometime it would seam weak as if the battery was low. Most times it would start normally, repeated times in a row, and then do just as you describe. This went on for months. After checking all obvious connections, many times, I was puzzled. I then removed the key switch and the start button, from the panel, removed each wire and sprayed them with electronic cleaner after brushing them clean. No problems since and it has been six months now.