I googled it and hit this:
I'd say do the following:
- Check all vacuum hoses, esp. to the turbo VNT actuator from the N75 valve.
- Check/replace the N75 valve and wiring. As said above, they are not expensive.
- Try manually moving the VNT actuator up and down. On a 1.9, that's best from below with the undertray off. I *think* that if you have VAG-COM handy and do an output test you can drive the N75 (and so the actuator).
Which is pretty much what is said above. All of those are easy DIY things. Dropping & cleaning the turbo a bit harder- and I've never done it so can't comment.