Is there any chance you might have accidentally been running two instances of the Navigation software? I'm not sure if the latest version now prevents that from happening, but I think when the 720 first came out it was possible to launch the Nav app, then go back to the main menu while leaving the Nav app running in the background. If you then clicked the Navigation button on the main menu, it would launch a second instance of the app, and problems like you described could happen.
If you went straight to the Nav app after powering on and then stayed there the whole time, then two app instances running together would not be your problem....