Yamaha 50 hp 4 stroke missing at high revs

[Brasdor Boater]

I've noticed recently my 2008 Yamaha 50 hp 4 stroke will miss at full power. This miss/breakdown will occur after the boat has been running full throttle for a while - 5 to 15 minutes. I can feel the miss and see it on the tach. Normally runs full throttle at 5300rpm. When breaking down the tach bounces between 5300 and somewhere just below 5000. If I back off on the throttle the engine will run fine at 4500 rpm to use an example. Occurs to me something is failing when hot. Any suggestions?