C80 map question

For the past year the map contour on my C80 has had square edges and is offset by as much as a mile. I tried a new navionics card and have gone through the menu too many times but can't fix the problem. Is there any way to solve this without having to upgrade to a new unit?
I was told the unit is probably reading the base map and it is either a setting I can change or the card reader has a problem.