I have a 2.2 tdci st and it felt like it was surging and when you nailed it in 4th,5th and 6th it felt like the power was coming and going,I put 20k boost straight into the turbo and fuel system cleaner and it's pulling fine now with no surging.
Funny thing was I was getting 48mpg after I serviced it then changed the tyres for new uniroyals and I'm getting 43mpg????