I would check for intake air leaks and try to see if turning in your idle
screw doesn't help (sometimes air is leaking past a screw that is turned out
too far).  If you can't find anything this way then it could be your idle
stabilizer unit.  I have these for $50 if you need one.  (this is for 1.9l
Vanagons only, the 2.1l idle stabilizer unit is $199).

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242