ford mondeo 2001 new shape 2ltr zetec estate engine cutting out?

while driving, after about 15 miles the engine will die to a tickover if the throttle pedal is depressed too much(above60-70 miles an hour) i am unable to get power back and ultimately end up by the side of the road, the engine is still tickking over and when i rev it again it revs up normally, so i drop it into drive and off i go again, within a mile or so it does the same thing again, if however i drive the car at a constant 45 mph or under as long as i dont try to accelerate too hard it is fine (but does annoy drivers on the A13! )
Please, any ideas would be gratefully received.
Andy.