We recently bought a pilothouse that has Nobeltec TZ as the main navigation plotter with radar and AIS overlay. While bringing the boat back home the boat position icon and along with the overlay would change to another area. Some times it would lose all except chart and then come back to normal. Had to center boat numerous times. Appears it is getting a bad GPS position and losing the signal momentarily. The GPS position comes from an old Furuno NavNet but it appeared stable and not fluctuating. Any thoughts?