I can understand how you feel I had a very similar experience with my Voyager 3.3 Auto - Neither the RAC or 2 garages could resolve the problem. Possible cause revolved around the car's immobiliser system and solutions included disconnecting the positive terminal on the battery for a while and changing the shutoff relay neither of which seemed to resolve the problem.
Although It may have nothing to do with it, but one morning i decidied to use the spare key in the ignition and whether or not it reset the security system I don't know but the engine burst into life and I haven't had any problem since
