Vehicle Speed Sensor...I think. When your gps antenna cant get signal, the vss signal will still move your icon on the screen; it also works with the gps antenna to make the navigation more accurate.
Did your nav come with your vehicle? I installed my oem nav myself, so I had to buy the vss kit off of ebay. Mine is a brown connector that plugs into the port on the far right if your looking at the back of the nav. It has one green wire coming out of it that goes down and splices into a wire in the junction box by the brake pedal. Maybe the wire at the connector is loose or possibly at the junction box.
I am not sure if this is your problem, but seems like most people that have nav problems fix the issue when they start messing with the vss wire. There should be a ton of information on this site about the vss kit.