Did you go into the menu and turn off the depth, then turn off the entire unit, turn the unit back on, then turn on the depth again ?
If that helps, good. If not, the next best thing to do is call Lowrance. M. thru F. 8:30AM to 4:45 PM CST @ 1 (800)628-4487
They may have you do a "Factory re-set", as that seems to clear up most anomalies.
Good luck. Would you please post here the results ? Thanx.