Intermittent loss of power - GT TDI - BMN engine (170bhp)

[.Ben.]

In the last month my car's developed a problem when cruising on a light throttle @ 70-80 ...
 
Under normal circumstances (hard & gentle driving) it's great but intermittently when cruising, if I want to accelerate or maintain speed up a hill, I get literally no increase in speed or revs until I lift off the accelerator. I then have to progressively feed the power in to speed up.
 
I've got no dash lights or loss of mpg. However, I did have it scanned and have a VCDS code:
 
P0299-006 - "control range not reached" - Intermittent
 
I've read multiple forum posts that point at:
 
- Faulty Pressure Converter Valve / Boost Control Solenoid (N75 valve?)
- Perforated diaphragm in the Vacuum (VNT) Actuator ?
- Faulty Intake Manifold Flap Actuator
- Perished/split inlet manifold gasket
 
Anyone offer any advice on which is the most likely to be causing this fault?
 
Thanks.

[.Ben.]

For anyone else that has this problem, or is interested, it seems to be a fault with the diaphragm in the VNT actuator on the turbo.

The actuator is a sealed unit and - although it's a bolt-on unit - is not available as a spare part. Apparently the reasoning is that the diaphragm has failed due to sticking turbo vanes and replacing the actuator doesn't deal with the root problem and will fail prematurely.

So.... New turbo time   

I found this : Garrett GTB17V.

Hooray.