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.
Bob