[Dave R]

I strongly suspect you have leak on the suction side of the raw water pump, on a part that's under water if you are going slow, but above water if you are on plane. When under water, it's just another water inlet and the boat will run fine. On plane, it allows air in and since air is vastly less dense than water, this will dramatically reduce the amount of water getting drawn into the pump and the boat will overheat until you slow down and put the leak under water again. The most common place for this air leak to occur is on the end of the hose in the gimbal housing. The hose gets clamped to a plastic nipple and the clamp is loose or the nipple is bad.

When the boat is out of the water, remove the water pump inlet hose and put a garden hose in it. When you turn it on, water will come out of the inlet holes in the drive, but it will also escape from the leak and help you find it quickly.