When not in guidance mode (i.e. no route planned) does the symbol for the satellite in the top right corner of the map have any dots around it? If not then there are no satellites being picked up and the Nav is using the wheel sensors to measure it's position using dead reckoning. Id guess that the reception is intermittent, possibly a faulty wire, otherwise it would not have picked up your new location when you installed the Mk IV.

If there are regularly no satellites when the car has been in the open for at least 15 minutes, then the easiest way to check the GPS antenna is to get a cheap one off e-bay with an SMB connector, something like this http://cgi.ebay.co.uk/GPS-Antenna-amplifer-SMB-M-Magellan-Trimble-Lowrance_W0QQitemZ250052420941QQihZ015QQcategoryZ4 668QQtcZphotoQQcmdZViewItem connect it up to the back of the Nav and see if it makes any difference while the car is parked in the open.

You can also check the GPS receiver state from the hidden settings menu (Settings menu and press and hold the menu button for 10 seconds).