Cracking sound and slight loss of power

[Elton]

Hi,wondering if anyone can help on this matter,been having this problem with my 2007 Transit 260/130 since about January,at first it was a very small lack of power when the turbo kicked in,more like a turbo lag which got progressively worse as time went on,at the time it was still under warranty so was in and out of the dealership on numerous occasions trying different things but with no success then last month the turbo started passing oil so had to replace the turbo but as the warranty had ran out,had to foot half the bill myself,thinking that it would eventually be the end of the problem but NO,there is still a slight lack of power and a kind of cross between a pop and a bang when the turbo kicks in which seems to be worse when cold!
Please help as the dealership can't seem to find where the problem lies and i am just about at my wits end!
Thanks in advance

[AndyG]

Have you had the fault codes read
How many new EGR valves have you had
Has the latest calibration been installed in the ECU

[Elton]

In answer to your questions,yes-no codes,no EGR replaced and yes-new callibration installed.

[AndyG]

Then, as it's such a common trouble maker, I'd be looking at the EGR valve