If it were mine, one of the cheaper things to look at would be the idle control valve (on top, up front, to the left of center) - these can get gunked up, and a vigorous (and I mean vigorous) cleaning can be helpful.

Here is a picture: http://catalog.eautopartscatalog.com/mercedesshop/sophio/image.jsp?title=Idle%20Control%20Valve&url=http%3A//img.eautopartscatalog.com/live/C403011604VDO.JPG

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D