I have the same issue, but it goes away after a 15-30 minute drive, that is, the system correctly locates the vehicle. I have never unplugged the antenna cable, so I do not think this is the cause (although it might be). In my case the problem appears as in the following situation.

I drive through long highway tunnels where the reception is lost. If the reception is not restored soon after exiting a tunnel and the map is not very accurate at that specific location, the navi system attempts to calculate the position of the car assuming some nearby location (on a road) and following the car movement sensors. This completely disorientates the system to a point that it reboots itself (after 15 minitues or so) and the problem is corrected.

The higher my road speed, the more easily does it lose the correct location and the more it takes to restore things after reception has been restored. As I said, it rarely takes more that 20 minutes at moderate speeds (80 km/h or so) to correctly locate itself again. The more difficult it is for a gps to operate properly (high buildings, cloudy day, etc) the worse it is. In a city, usually the time it takes to wait at traffic lights is enough to correct its position once connection is restored.

Hope it helps...