Help

I have a 95 kawasaki 750 ss jetski with a small problem. It has been to several dealers for service, since it was new the jetski has set since 96 none of the dealers has been able to get it to run right. It has no hours on it. I bought it from a friend that owned a dealership last year I put a new top end in it which it didnt need but i did it anyway i pressure checked the crank seals and put it back together the jetski, I even put a different set of carbs on it. I have got it to run the best it has ever ran. I pluged the crankcase drain also. It will run great for about a half hour then it wil bogg out let off the throttle and it will take off again for a few seconds and do it again. I have tried to hit the primer loosened the gas cap and it continues any ideas??? Its more frustrating than its worth but its a challenge that i havent gave up on.