For eight years I have had a 1997 Mercury left-hand drive station wagon, 3-litre V6 with automatic 3-speed gearbox plus overdrive, which is also automatic. I have been meticulous with services. All of a sudden, whereas the gearbox changes beautifully in the 3-gear mode, the overdrive no longer operates. Mileage is 67,000. Do you have any ideas please? The car was built in Canada and has been superb to date.