Ford Focus fluctuating power loss

Recently I have experienced problems when pulling away and when accelerating. When the engine is just under 2000 revs the power from the engine seems to fluctuate and only after 2000 revs does the power come back slowly. If I put my foot down when joining a motorway the car seems to hesitate and splutter (no sound) just as if power is coming and going. I first noticed this occurrence in 3rd gear but it seems to affecting all gears depending on the amount of revs??