First post. Welcome!
Bad prop or the engine is mounted improperly (probably too high) are the most frequent problems. There's a rubber hub in the prop designed to protect the drive line on hard impact. Occasionally they start slipping when a lot of power is applied. Not hard to troubleshoot. All you have to do is mark the prop nut and the prop hub. Then you go for a ride and see if those marks are still lined up after one of the little episodes you're experiencing. If they are lined up, the prop is good and your issue lies elsewhere. If this engine install has been in use and just started doing this, it is much less likely a bad engine install. There was one guy that had a tree branch caught in the bottom of the boat just ahead of the engine. That might be worth checking too.
