have they checked the coolant temperature sensor?? It might be on its way out, if it fails it could be telling the computer that the car is operating temp and therefore leaning out the fuel mixture before it should.

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD