+1 The wastegate servo is serviceable, as are the vacuum elements that control it. Sometimes the wastegate itself becomes stuck, and 50/50 you can free it up on the car rather than having to R&R.

Actually, that assumes (always bad) that the problem is low turbo pressure. The most common problem on these engines is a clogged pressure sense take-off. Look on the left side of your intake manifold. Find the large sensor. Just below that, you'll find a hose connected to a screw-in fitting. Remove the hose, undo the fitting and clean it out, replace with the hose. I bet you find your mojo. Mechanics these days have forgotten all the oddities of working on old diesels.

If you want to do a pressure test, just attach your pressure gauge to the fitting described above. Easy.