You are running too much prop from the sounds of it. What are your MAX WOT RPMS? Also if your outboard is too low on the transom you will have this issue as well as others have stated too much weight in the stern of the boat will also cause it. Check the engine height on the transom first then prop correctly. Personally I think you would be better with a 21 Temptest Plus prop on that setup. You can contact Ken at
www.propgods.com and he has a prop swap program that works out well for people. He is there to make sure you get the correct prop for your setup. You can tell him that I sent you.