2.5 TDi V6 - New turbo = underboost, then overboost

I have just replaced the VNT-20 (GT2052V) turbo on my Passat (MY 2002) it is 2.5TDi V6 after a faulty actuator with blown diaphragm.
Now I have underboost and dissappointing acceleration until about 3000rpm, where there is a sudden boost beyond belief. When I hooked up VAG COM, there are no fault codes.
I got some boost actual vs. requested graphs which should be below, and there is also a link to the same pictures on flickr which can be viewed full size, they show too-little boost until around 3000rpm, then too-much. I have tested the actuator with vagcom and it moves excellently, and the VNT mechanism is free.
I have checked all of the vacuum pipes and they seem OK, the N75 valve is only 9 months old, and I don't suspect it because it moves the actuator with VAG COM in 'basic settings'
I can't think of anything else to try, the only thing I can think of is that my old turbo had a G on the end of the part number and this one an F.

Green - Requested manifold pressure
Yellow - Actual manifold pressure (lower of two pale lines)
Gray - N75 duty cycle (higher pale line)
Red - RPM





http://www.flickr.com/photos/13666355@N00/sets/72157600156139841/

Well, this is terrible. I have been running more tests etc...
I replaced the N75 valve, and this made no difference.
I have tested the EGR valve, and it holds vacuum OK, and makes a snapping shut sound when vacuum is realeased.

I have just shortened the VNT rod quite a but and it is a lot better at low rpm, it has less of a laggy feel, but it won't rev over 3000rpm, it sits there smoking and juddering slightly until it changes gear. If I hold it at this point where it goes flat at 3000rpm, and quickly let off the throttle and straight back on, it will suddenly overboost with a big surge of power to the red line. Sometimes it is also sluggish at low RPM, such as if I stick the tiptronic in 2nd and slow down to about 1500rpm, and then press the throttle, hardly anything happens.

I am sick of this car as it is so hard to fix. I have ordered a non-return valve for the vacuum lines too.

How does the VNT actuation get controlled?
Is it a value from the ECU map? or is it calculated on the fly?
When I first set off in the car, should the VNT move to maximum boost position? It seems as though boost is more a function of rpm than n75 looking at the graphs.

I am wondering if either the new turno is a dud, or the VNT system is faulty in some way. I am thinking of getting another turbo from a more reputable source.