115 PD boost problem

[georgezippy]

I'm almost at the end of my tether with this one.
I have an 2000 Audi A4 PD115. Earlier this year it started stuttering/hesitating when under full load at low revs. previously it's low rev (1700 upwards) pull was simply astonishing, now it's surge/less surge/surge/less surge with a frequency of about half a second.....
I have read lots of forum posts about this issue and cannot find this exact fault described, especially as:

I have no engine warning light, and no fault codes given.
The car is not in limp home mode as it still has reasonable go.
Switching off and on makes no difference, it does it cold and hot. Fuel economy is normal.
The stutter is very specifically from about 1600 - 2200 rpm, above this it's normal.
General dawdling/driving the fault is not noticeable.
It's still capable of 3 figure speeds when I tested it on a friends private racetrack, no stuttering whilst doing this either.
At no point has it stopped working.

It's had;

4 new injectors
new fuel filter
new N75 valve and MAF
New head wiring loom
New clutch and DMF
All vacuum hoses replaced.
Boost pipes checked ok.

All this has cost a fortune, garages said this was what to do and no difference has been made, the new injectors perked it up a little but didn't fix the fault.
Where to go now, wouldn't an EGR or turbo fault throw a code?
I want my grunt back. (without this shuddering)

A V6 petrol would have been cheaper to run after all this.
Help!

[georgezippy]

Thanks for these suggestions. I did various vacuum tests with a mityvac pump. It held vacuum well, but I added some pipe clamps around a couple of connectors that came apart a little too easily.
The turbo actuator seemed to move well but I took it out to make sure. Worked it with the pump a few times and coppergreased the spindle. Thought the lever on the turbo felt too loose.... could it be snapped? Anyway I put it all back together again (apart from the circlip... how the hell do you get that back on?)
Pulsing gone, full grunt restored, maybe the actuator was a bit sticky and unable to control boost properly. Such a simple fix, and worth remembering for anyone else with boost control faults. None of the professionals I took it to considered looking at that.