Honda 50HP- 4 stroke- need a little Help.

[wonderboys]

Hello, I have a Honda 50 HP motor (4 stroke) 97'. I had a problem with it this weekend on the lake. I coasting up lake with throttle at a steady speed, and then all the sudden I heard the engine rev up loud. I backed it down and then pushed throttle forward. The boat would barely go. So I thought I "spun" the prop. I went and had a new prop installed, boat then started working fine. But every once in a while when I have the throttle at a steady speed I hear the engine rev up loud. When it does this I just back the throttle down and then go forward. The boat will then go forward like nothing is wrong. It never had rev'd up like this before and I am curious on what the problem might be. Thanks for any help.