Sounds to me like a tired pump or a cut impeller.
There are a couple things to consider when dealing with situations like this...
1. Is your tachometer accurate? You would be surprised how often that happens.
2. A worn pump will normally act a bit different than a small impeller, a small impeller will accelerate more slowly than a worn wear ring, a wear ring issue will pretty much snap to a higher RPM immediately.
3. How does the boat ride? bow heavy?, bow light?, plow? Another possibility is a serious cavitation problem and improper loading, this is harder to troubleshoot. I will say that trial and error play a huge role in figuring things out, but it is a lot easier to do when you know things are "proper" as far as pump condition etc.....
Good luck...
GT
