Have you checked or had checked your EGR valve?? its on the top right of the engine as you look into the bonnet.
I have had problems with it sooting up and not closing 100%, this then lets boost pressure leak past into the exhaust.
If you plant the throttle you get a lag (most apparent if coasting upto a junction in 2nd and trying to pull out whilst still moving, the car feels really gutless) once the revs get upto around 3K the car pulls again, its because the boost pressure overcomes the leak and the car then revs ok.
Only a tiny gap, with it not shutting can cause this problem, my brother (a mechanic) solved the closing problem by soaking the metal part of the valve in diesel for a few hours then cleaning it up. Refiitted was like a new car, more responsive.
The valve is actually fairly easy to remove, just a few bolts.
Mine never threw a warning light up, prob because the gab was very small, say 3mm, expect if it gets much bigger then the ECU would be notified by the valve, as this fault can throw a light up.