Bob, I dont think the issue lies in the LU. I have had two outboards that have had dog clutch issues, and in both cases the motor would rev up when it slipped. On one of them (a rental boat) I ran it till the LU blew. In the rental when it was happening I had initally thought the motor had a dead miss, but it shook the back of the boat pretty violently and then after a few start and stops forward gave up the ghost all together. In all instances the motor reved up when it would jump, not sputter down as yours is in the video. I know you said you ordered new plugs, and after looking at the ones you removed I would start there. The corrosion on the exterior of the two plugs indicates they were getting water sprayed on them. The water alone could provide a path for the spark to short out, with the level of rust on the exterior of the plugs it looks like its getting a fair amount too. The most basic in any trouble shooting is to start with what doesn't look right, and those plugs do not look good.