A common problem on VW TDIs is with the Mass Airflow Sensor sending a code to the computer that its readings are not correct for one reason or another. Although it is not recommended that a cleaner be used, some have had success in extending the MAF life using a variety of cleaners.
MoGolf posted his suggestions as to where to get a replacement MAF (prices 5/2008).
World IMPEX - $115.00+shipping
TDIParts.com - $119.95+shipping
1stVWParts - $153.72+shipping
Others have spent up to $300 for a new MAF ... most Volkwagen Dealers are going to be on the high side. There are a few chain part stores that handle Mass Airflow Sensors for our VW TDI, but beware that often a cheaper 2.0 L gas engine VW sensor has occasionally been sold as a TDI version. If the prices are substantially lower than those above, shop elsewhere. Same for Ebay listings.
Advance Auto Parts has a listing - screen shot below 5/16/2008:
Autozone had a single listing on 5/16/2008 -- screen shot below:
Aaron M. (dcdog59) purchased MAF 5/20/2008
Advance Auto Parts for $111 no shipping -- the part itself said BOSCH with the part number ending in "C"
MAF for TDI
The MAF at Advance autoparts has a limited Lifetime warrenty so it should be the last MAF you need to buy!
Looking up the MAF on Amazon one place is selling it for less then $90