Defo not the box, most likely thing to be is the speed sensor, autos use more than one, there is one that gives a vehicle speed to the ecu, this is nothing to do with the speedo generated signal, but when this sensor fails the car doesn't realise it is still coasting and raise the revs slightly, the zetecs do run very lean mixtures so could be a air leak if no joy from speed sensor.
tabetha