Terrano gearbox keeps jumping out of overdrive and

Nissan Terrano's have an Electronic Computer that controls the gearbox. It is hidden behind the left hand rear Speaker box, right hard up against the rear seat back rest. This system controls all functions regarding the gearbox and it's rev rates. If your vehicle is shifting back on long drives then it is possible that the computer sees any slight increase in revs as a desire to "Kickback" the gearbox to a lower gearing, hence it dropping out of overdrive. Unfortunately a gearbox specialist with a computer "Plug-in" is needed to fix this, but it is fixable. This problem should not be too expensive in comparison with a new gearbox.