This has plagued me ever since I bought my XJS. It could just be a loose wire. Simple checks are to try disconnecting the battery for ten minutes and also try changing the gearbox fluid. It may be the Auto ECU which can get water damaged. These can be changed easily, and you can get reconditioned ones for about £30 on e-bay.
Havind said all that, despite all of the above I still can't get mine to bl**dy work...
Havind said all that, despite all of the above I still can't get mine to bl**dy work...