I had the same problem a few years ago with my RS Turbo, It was in and out f 4 different Ford garages and one of them which was a main dealer stripped the engine down but couldn't find the problem,they said its probably a faulty manifold casting. I put up with the problem for several weeks and then decided to take the petrol tank off and clean it incase I had bad fuel or water in the fuel. When re-fitting the tank a tiny drip caught my eye, it was on a plastic fuel line connector under the midle of the car,. I remembered reading in a Haynes manual that even the slightest leak on a fuel injection system can cause big problems. I cut off the connector and replaced it with a 50mm piece of fuel pipe and two jubilee clips off the lawn mower, Problem solved. The Moral of the story Is 'Check the simple things first'.