F 90 fuel problem

I have a 2006 90hp on a premier pontoon boat and when I go to full throttle of 6400 rpm's I sometimes get a hesitation and the rpms go way down and then jump back up and the boat starts to hobby horse, If I back out of the throttle the problem stops. I can run at 4000rpm's all day and have no problems. This only happens when I am at full throttle. It seems like it is starving for gas at full load. This has been going on since the boat was new and the dealer told me that I just needed to pump the ball fully before I start the boat. Any help would be greatly appreciated.

[boats.net]

Probably is a fuel problem, try pumping primer bulb when it looses rpm's to see if speeds back up. May need to clean carbs. At 6400 rpm's it should be hitting the rev limiter, if tach is reading correctly may need to put a larger pitch prop on it.