I have a 50hp Mercury on my pontoon boat. While running at WOT (approximately 5000 rpm) the motor suddenly dropped to around 3000 rpm. It happened a few times this weekend, but cleared up and continued to run fine. But this time it would not recover. The motor will start and idle fine. Full power in reverse and revs up fine in neutral. Changed the fuel filter and checked the spark just by removing plug wire and listening for the slight change in engine speed. All that seems fine. Any ideas on what else it could be?