On my old boat, it was voltage drop due to the size of the wire used from the key switch to the solenoid. As it aged, its ability to support the necessary voltage degraded. This apparently is a common issue with Yanmar engines to the point I saw a 'repair" columnist in Sailing magazine discuss it. His solution which was also discussed on Sailnet previously was to install a second solenoid. I'm sure if you search sailnet, you can find the discussion and the details.