it's more than likely your transducer position. as your hull picks up speed , it generates a lot of turbulent water. the transducer has big problems recieving the signal through this turbulent water.

i had to remove mine , take the boat out for a spin to find the cleanest water leaving the transom and place transducer there.

as you look at boat from side on , the bottom face of tx should be in line with hull to slightly lower in the water . the rear edge of tx lower than the front edge.

Grayzeee is spot on, there is turbulence happeneing under the face of your transducer once you get moving.

Sometimes it can be cured by just changeing the angle forward or lowering it down but lf this fails do as he says and take it for a run and see where a clear strem comes through from under the stern and place it there.

I have had a lot of trouble with my tranducer over 10knots picture is crap .so I took the transducer of the back of the boat and put it in a bag full of water .Jesse held it in place in the back (bottom) of the boat . clear picture at any speed .So im now making a wet box to put it in .I have tryed and moved my transducer a number of times and it didn't matter where I put it on the transom I could not get a good picture.