your paddlewheel could be sticking, they ocassioanlly go out. Has the boat always had the problem? If the paddlewheel is in the wrong place it could be getting turbulance from the skegs??? Just a thought. PP Crusie is RPM based I believe, it may not hold speed very well as it's tuned to the boat RPM not speedo.
If yoru analog speedo has a pitot tube it may be clogged or the pickup is cologged, just take it off (pickup) and clean it out.