Perhaps the mass air flow sensor.

I would suspect one of the following, in this order:

1) one or more of the vacuum hoses that carry vacuum to the turbo VNT mechanism or EGR is collapsing due to age or kinking due to bad routing; the hoses need to inspected and replaced if necessary. They are cloth-covered so any fraying or wear should be fairly easy to spot.

2) the N75 solenoid which controls the vacuum supply to the turbo's VNT mechanism is sticking and failing; the only reliable test / cure is substitution.

3) the MAF sensor is dying (but this would tend to throw a Check Engine light, I think). Again, the most reliable thing to do is to replace it.

Could be the turbo VNT mechanism. reasonably common on Golfs.

Try myturbodiesel.com good website to have a browse on.

would be worth getting any fault codes read. Faults can be stored without putting dash check light on.

You could try using Torque app (provided you have an Android phone). You can use it as a diy code reader and also monitor live data as you drive to see if that yeilds any clues.

Have a look

www.honestjohn.co.uk/forum/post/index.htm?t=114789

Start running it on 'super' diesel. Not just for a tank or three, but consistantly.

As suggested, an Italian tune may well be beneficial.

Try using Torque and moniter the turbo boost. If it is the variable vanes sticking you should expect to see the boost fall off. Also watch the boost rise and fall with throttle input - it should be smooth and follow the inputs. If not, then suspect the vanes.