Probably no use, But I had the same thing with an old Marina 40.
The kill button was missing when I bought it, so I wired in a kill switch by the helm.
The vibrations of the engine had worn the insulation on the lead and was shorting out.
It did not affect it too much at speed because the contact was being made and broken.
When in neutral, the engine sits in a different position and does not make the connection.
When in gear, the engine casing moves and sits in the damaged insulation.
I hope this makes sense. If you have a remote kill cord, check the lead.
Jim