Something big that require revs / centrifugal force is the prop, and one blade sticking will obviously cause balance issues. I actually wondered if you had a bucket wrapped on the prop. If it was a shaft, something caught on it. But you're saildrive.
I know you're in selected neutral but I wonder if your Morse control or cables needs adjustment and the gearbox is actually engaged or part anyway. So I would start at looking at the prop when it's doing it, disconnect the cables and check the gear select position and after that if that says nothing, start thinking about gearbox to damper pl