What "I" would do (especially if it's a newer unit) is call Garmin and speak to a tech explain it all and see what they say; IF it's a software issue , they should be able to fix that as well...

I know on some Lowrance units they had some software issues with shallow water readings and you just had to download the beta version, put it on your card, insert in unit and turn on... not sure on Garmin units though, (sorry)