2.0 hdi belingo ?

Hi i have a 2.0 hdi belingo it is a 02 plate.the problem is there is a lack of power when pressing the accelorating it is really sluggish but if you lift your foot off the pedal and press back down slowly it takes off as it should.i have changed fuel and air filter there is no black smoke.when pulling out of junctions it pulls away really slow but the revs are where they should be.any ideas please help