A new set of plugs never hurt (except in your wallet) and may help. Don't be surprised if it doesn't go away.
What you could be experiencing is the transition point from stratified to homogeneous mode. They all do it. Some you really have to search for it and on others it is more obvious. This is a normal condition. You can adjust the throttle position sensor which will change the rpm at which this occurs. I wouldn't mess with it. The boat is usually plowing when this occurs anyway. Mine does it between 1400 -1800. Disclaimer: I know just enough about outboards to get myself in trouble.