2010 Defender TDCI problem

I have a defender 90 TDCI and have been having a few problems! It started to run really rough last week as if it was running on 2 cylinders. I previously had a new EGR Valve fitted as the other one was playing up. I then had a new pressure switch fitted in the rail as the pressure was up to 85 rather than 20 (give or take). Although when replaced, the engine still ran the same. It was traced back which revealed that 2 of the injectors was dodgy. So had them out and replaced them. All is well again. But now I have a new problem. When you start the truck after around 4 seconds you can hear a distinctive change in note. It seams like something kicks in where you can then here the noise through the air filter intake in the side of the wing. Also when driving along, if you hold the revs at around 1750rpm and then try to increase your speed, she seems to stutter a bit as if it is not sure whether to go or not! (Lack of power). Although when you put your foot down there is no problem. Any ideas please!