As above, it could be sticking flaps in the turbo but another possibility is a split or collapsing vacuum hose. The VAG diesels I have found in the past are very sensitive to any sort of split in a hose or when they come off.

A common thing that used to happen was the hose connection on the vacuum damper (a spherical shaped thingy) at the front of the engine used to break and cause poor response and lack of power like the turbo wasn't working. I've replaced a few of those in the past.

Check every little pipe you can see carefully, some of them are a little tricky to get to but are worth checking!