I have seen a few erratic faults related to the auto transmission caused by poor earthing to the rear light clusters.The rear lights appear to function ok,but there is a very slight voltage drop on the system that causes problems with the gearbox.Remove both rear light units and check the multi-pin connectors attached to the bulb holders.If there is any possible evidence of poor connections or an overheat /burnt terminal,I would run new earth leads directly onto the bulb contact plate.Certainly worth a try.hth.

Thanks for both the above posts. Yes, you were correct. One bad earth on the rear light cluster and problems solved! Can't thank you guys enough.