my nav seems to be off by about 1/4-1/2 mile. i pass a street that doesnt show on the screen, then, a little further up i see the street on the screen. it just started happening...has worked flawlessly in the past. i have the most recent map setup as well. i will try to clean the disc and see if that helps, but any other suggestions...or has anyone run into this before? i called our radio exchange company, and the want $650 for and exchange unit.