My depth on my Humminbird 859 DI is messing up in the 2d sonar. The image on the screen is correct, shows baitfish and fish like always, shows the bottom and had the depth lines on the screen so I know pretty close the depth. But the actual depth number is going haywire like im in a foot or water or less. It will say its anywhere from 0.9ft-71feet and constantly changing even though the bottom on my screen isnt. Its mounted on my trolling motor.