throw the nav unit into a refrigerator, reconnect and see if it will read the CD... i suspect a "cold" soldering joint but i could be totally wrong... when the TV module generates the menu, it means that the nav has fallen off the ibus and the tv module takes over... falling off the ibus can be directly firmware related (could be due to some hardware made different manufacturer not fully compatible - is this unit same as the other ones you are working with?), or could be something simple and stupid as a loose joint...

oh... and about the refrigerator - i was just kidding... don't cause condensation damage to the unit!! lol...