Rover 400 Diesel Problem?

Does anyone here have any clues as to what I can do to fix this problem .. I have for a while now been having problems at low revs my rover 420 simply looses pulling power .. if I put the engine under load how ever (ie if I quickly re push the peddle harder) it picks up its speed.
I have had an engine diagnostic and nothing found I have put a new air mass flow meter on and still no change.
Im looking for some Rover know how here if anyone out there has suffered any thing close, any help would be so very much appreciated.