[DaveNP]

Starting and immediately cutting out sounds like the antenna ring and immobiliser to me too, either the antenna ring around the ignition key hole is failing or the electrical connection to it is failing. Some people have had success just by opening up the steering column shroud and unplugging and replugging the connector a few times to clean and reseat the contacts. If it does need a new antenna ring they're not too expensive and fairly easy to swap.