it does sound like you might have a transducer issue (perhaps a fault or shielding/interference). one thing to try if this happens on the water is change the display from auto vertical range to a fixed range. you may find that the sonar response and display of fish and the bottom is just fine, the only fault is the unit deciding what the depth is and displaying that number.
next thing to try is power down the unit, disconnect the wires, reconnect then, and power up...
FYI- on GP, I am normally going to set my display to show 20' to 80', then zoom in from there as I confirm where the schools of kokes are...cheers, roger