Playbouy Boat Honda Motor Problems?

We bought this pontoon new 2005 playbouy fish and fun 22 foot. 50 honda motor ever since we bought it we take it out on the lake and it will run fine then it over revs and the motor gets louder. it will be at about 5,200 RMS then it will shoot up to 6,200 or more RPMS. We have took it to the dealer 10 times and they dont no whats wrong with it any suggestions?