I am having a similar problem although mine actually stalls.I believe it is related to the Air Mass FLow but have a few more tests to run. Off course mine is on a different engine style and older and simpler 560 so they may be completely different.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)