I would imagine it might have something to do with water turbulance? Do you have a sensor thats located at the bottom of the boat somewhere? In my grandpas boat, the sensor is at the rear bottom near the hole where you can drain the water, if your going very fast it doesnt work very well, slow or stopped and it works fine. If you have that type of sensor make sure whatever holds it is tight so the sensor cant move around.

either that or you got some funky wiring issue lol