[SD40]

Okay so,
I have had my S40 (2008 2.0 diesel Lux) for 18 months, a few problems along the way, but I love my Volvo.
About a couple of months ago I noticed an occasional and slight surging in power when accelerating hard in fifth and sixth.
It became worse, I changed the fuel filter...I had noticed air in the clear fuel line and thought it might be the cause. It seemed to cure it, so I thought that I would fit the spare cone air filter I had lying around. Fitted okay and tried it out, I heard a lot of unpleasant induction noise and the performance seemed sluggish and dull. I returned the intake to original but the performance didn’t really come back.
I obviously checked the intake system but found no problem.

The car went into limp mode showing an underboost code.
I cleared the code and it didn’t return.
However it feels like the engine is not producing boost. I bought a WiFi obd2 interface to use with torque pro, but it only worked a couple of times...much to my annoyance, so I haven’t been able to check the boost conclusively.

This is where the sore head begins.
I will condense the story, but suffice to say, that I have trawled through almost every thread on every forum trying to find a solution.
I have changed the inter cooler, visually checked the entire pressurised and vacuum sides of the engine system.
I have replaced the vacuum solenoids.
Replaced the MAP and MAF sensors.
Checked the throttle position sensor.
Used two treatments for sticking turbo vanes.

Its a lot of info, but hopefully someone can make some suggestions.
As I said initially, I know that there are other posts about very similar problems, but not many solutions.

Thanks for any help.