How to fix Vauxhall Combo 1.3cdti Turbo?

I have a combo van 1.3 CDTI 2005. it has a lack of power from 2k-2.5k rev range. its like the turbo is not kicking in when you put your foot down. but if you apply a little bit of pressure to the accelerator peddle the engine eventually gets there the when you get past 2.5k its fine and works. is like something is restricing the engine almost like limp mode. i plugged the tech2 in and got a map sensor fault. think it was low or faulty pressure. i cleared this and went for a drive with no difference at all in the engine plugged the tech2 back in and the fault was back so iv chnaged the map sensor with no joy