I have a 1998 force mercury 90 hp motor That reduces rpm to 3000 at high-speed, wide-open throttle. Sometimes when it's running at high-speed it will bog down and reduce it to about 3000. This of course lowered the speed of the boat . What could cause the engine to lower it RPM and not run properly? By the way, it runs fine at low speed's and Idles fine. If I disengage it from the propeller to increase rpm, it will hold steady with the throttle wide open