Help with a Humminbird 525?

[Medics-R-US]

I bought a brand new humminbird fishfinder 525 and it is giving me some issues.

It works great in water up to 35 feet but when I go any deeper it does not register the depth or show any of the bottom features at all unless I set the depth manually.

I was fishing in Lake of the Woods and had the problem there whenever I would go into deeper water. Occasionally it would work for 1-2 minutes but then it would always show that I was in 0.9' of water and it would have a gray line going from 0-2' on the screen with nothing below it.

Any Ideas or should I just take it back and exchange it?

I bought it at wal mart and I still have the reciept for it so exchanging it or returning it is not an issue.

Thanks
Michael