another thought just came to me, This seems like a problem that used to regularly occur on lots of Ford cars and it turned out to be the ignition switch electrics module that sits at the back end of the ignition barrel, so the electrical connector attached to the far end of where the key goes in but right at the back end. Its plastic and wears badly. cheap to replace and simple it just sits between the electrical connector and the rear of the barrel assembly. No one could find the fault originally until a chap came in and said if he put pressure on different directions of the key, sometimes push it in, others lift it or pull it down he could get it to start. we all initially thought transponder but it wasn't!