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It's hard to tell from that photo but it appears to me that when you lower the motor to the running position it is too deep.
You will find when you get it at the right height it will go faster and be easier to trim and control.
The "cavitation" plate really has nothing to do with cavitation...it prevents ventilation, which is the induction of air from the surface to the rotating blades just below the plate. When this happens propeller efficiency drops quickly. Cavitation is the "boiling" of water on the suction side of the blades due to the local pressure dropping below the vapor pressure. Generally this happens when you over load the blades. Ie by having too much pitch or rpm at a given speed of advance.
In any case you want see that plate just at the surface when underway. The reason why it is changing at different speeds is I bet the trim angle on the boat changes slightly.
In any case I think this winter you can adjust this so it is a non problem next year.
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