Lately my 1734c vx2 has been showing my bearing to be off as much as 90 degrees and after every sweep seems to get better maybe as little as 5 degrees and after running for 20 or 30 minutes it will be dead on except for my last trip bearing line stayed about 5 degrees off I also noticed it seems to be more off in rough water the targets are still nice and clear just shows the boat going in wrong direction any idea what's wrong?