Had a problem like this with my frst, the problem was though it was so intermitant that it never seemed to breakdown when it was on the rollers!! In the end (thanks to well lane!!) they identified that it was a wiring fault on the MAP sensor circiut. If what the other lads said above doesn't work, get a circuit tester and use the wiring diagram from ford or the haynes and tester all the links on that circuit. Mine ended up being some poxy inline resister/contact which is situated at the top right of the engine bay around the ecu. (It is a small black block with a 3 pin connector on it)- if you find it give it a wiggle when it's running if the problem seems to stop you need a new one. The part is not make by ford anymore, you'll have to scour the scrap yards!

Good luck hope this may help??
Darren.