Check to make sure the tranducer is mouted fully submersed when you are underway. It sounds like you have air or turbulent water around it. How far from the prop have you mounted the tranducer? you can get an awful lot of problems if mounted too close.
I agree with Fishy Finger, sounds like the transducer is in a turbulant area.
When you get it sorted, don't expect a fish finder to work very well at speed.
What you need to imagine is a narrow beam of signal being sent and recieved from your transducer and displayed on the very edge of your display.
If you were moving over a sandy bottom that over the space of 50 yards rises up to half the depth. If you were drifting this would show a gentle rise. Cover the same ground at 25 knots and it will look like a reef.
Also depending on the type of sonar you have the signal may not be fast enough at high speed to give accurate readings.
I had a similar problem with my FF last year. previously it would work fine at my maximum speed of 25 knots.
All of a sudden at 18 knots I got a blank screen, turned out the transducer had tilted up towards the transom a bit and was firing its signal astern rather than down vertically, fixed it in seconds.
As the boat drys out in the harbour I think the transducer must have come to rest on something at low water and moved it a few degrees
Mount the transducer inside the hull at a point always in contact with the water and isnt turbulent, as long as there is no air between it and the water it will work perfectly. Attaching it with a slow cure epoxy or mounting in castor oil will achieve this and avoid all your problems.