Hey Artmasterx,
Yea... I also had a TDI. The N75 valve is the way VAG controls boost. Theycrap out alot. The other problem is that the TDi engines can build up a ton of sluge inside the intake. Way more than ay D.I. will. I have seen some horror photos on the web. Seems that the newer Pump Duese w/common rail injection motors do thus more so. A lot of people put a plate between the EGR valve to stop this. And yes also check your vacuum lines for breaking(VAG uses shitty cloth lines that disintergrate over time) or being clogged with soot/goop. And... the MAF sensor is known to go bad too. casuing a low power mode. You can unplug the MAF to see if the car drives the same or not. It did not throw any codes on my car. But my car was a '99 115 PS PD model.
Also... IF you have a newer TDi with Pump Deuse - make sure to use the correct VAG oil. Just like BMW there are special oils for VAGs. 505.1 IF I remember right.
The only good thing about the TDi's is they can get 55 mpg plus (5 Liters/100kph)!!! The best I ever got was 4.3 liters and the worse was 12 liters/100 kph. I always averaged about 6.5L though. Its hard to drive slow with an unrestricted autobahn here.
Good Luck,
Dackel