Hey MD,
You did not say how the boat was riding, is the bow down or up? I am guessing that as you exceed 35 that the bow goes down so the whole keel is in the water leading to something I have heard called bow steer. I had this problem with an 18ft Sidewinder, got hard to control when I went over 37 or so. Solution in my case was to get a Place Diverter so I could trim the bow back up. You want to lift the bow up so so only the back part of the boat is touching the water.
The reason the bow goes down in the first place could be for a couple of reasons. First is the hull might have a hook near the stern, acting like trim tabs in the down position. Second is the weight distribution in the boat.
In case you are new to jets, a Place Diverter is an up down nozzle to give you power trim capability.
Hope this helps. Doug