I am now back home and was successful in replicating the 'malfunction' in both cars. When the CD is not in the player, everything runs like clockwork. When I insert the CD to the player the 'freak out' starts in both sat nav units. I understand it is hard to believe, but I am just relieved it is such a simple, albeit unusual cause. I would have never believed that a CD would make such a mess in the sat nav in a million years...