2003 90hp Johnson outboard 2 stroke studder at full throttle

[Matt2003]

I have a 2003 90 hp 2 stroke Johnson bomborder on a 20 foot pontoon boat. It starts easy, idles smooth, and runs good up to 3500 rpm. I fill the boat with premium gas before I put in the water every time. It has a studder anything over 3500 rpm but runs smooth at or under 3500 rpm and this will last about 1 hour. After 1 hour it will run up to full throttle and give no problems for the rest of the day unless I fill up with gas again. This boat has been at a certified service center for two weeks with no fix yet. I am afraid of the bill when I go to pick this boat up any help is greatly appreciated