2.0 tdi s-line 170 bhp running problems help because Audi main dealer CAN'T

Hi there my Audi is driving me nuts its been playing up and not running right for the past 2 years, I have cleaned the Egr valve out with no luck and it has recently been to Audi to have all 4 injectors replaced and it just seems worse! It will be fine for ages but then it's like a switch is flicked and I feel the car instantly change and lack power and hesitate at low revs, but when you accelerate it still pulls well but I can hear the exhaust loudness get considerably louder and the pressure in which the exhaust gasses comes through the exhaust is considerably greater too almost as if the bore size of the exhaust is being restricted right down! No engine light comes on and Audi main dealer cannot diagnose any problem but clearly there is! Also I have noticed if I'm driving for a while when it is running badly if I turn the engine off and try to restart it the starter motor turns over very slowly and the engine struggles to start, this is only started to happen recently as I used to turn the engine off before and restart it which usually makes the engine run better again! Please are there any idea of what is wrong have you had the same problems and how would I check if the Egr valve is working properly as its all electrical and doesn't run a vacuum! Thanks for any advise anyone can give me and I look forward to hearing from you!

Hi guys cheers for all your advice, I'm going to try changing the fuel filter first of all and see how that goes, then I guess I'll just have to get a new Egr valve as its been cleaned out drove fine for the first day then the next day straight back to how it was slowly getting worse! I've looked and as far as I can see my car doesn't run a PDF my down pipe runs from my turbo under the car and into the cat the rest of the exhaust is straight throw so makes it very noticeable when the car does play up because it gets very loud and on tick over it really gurgles like an old escort rs turbo with a 3" system on it, it's mad getting that noise from a diesel! If I let it run for a while on tick over gurgling eventually you can hear something click over from inside the engine bay not sure what and It goes back to normal and the exhaust not gets alot quieter! As I have read in a lot of these post the same is for me too at low revs between 1200-1800ish I get a juddering almost like a mis-fire but that's only done that since the injectors were changed and as per usual main dealer Audi are like well we cannot find anything wrong with your car sir we've had no experience with faults with the recall campaign of changing the injectors blar blar blar tossers! Also I have noticed the fuel consumption has been amazing, I keep looking down and thinking no way is it doing this kind of mpg used to only get between 45-50 mpg max now 55-60 mpg and the other night doing a 5 mile drive the mpg read 70 mpg and I was like whaaat no way! And it wasn't even like I was driving like miss daisy, guess I was doing between 40-50 mph all the way home, I'm getting soo tired of it all now nearly sold the car but it's such a nice car apart from the ****** engine throwing its toys out of the cot all the time, oh and the cam belt has been changed max69vk on 81000 miles car now on 92000 miles to be honest I don't know if they would have checked the torsion valve, is this something that still could be checked or will the Cam belt have to be removed?