It is probably your throttle position switch.  I have these available for '85
Vanagons (same as one used on 86-91).  I believe they are around $65 (not in
the office yet).  You should be able to move you throttle to full throttle
and hear a small click (engine not running).  Then allow the throttle to come
back to idle and hear another small click.  If not then your switch needs to
either be adjusted or replaced.  Adjusting and testing proceedures are in the
Bently Manual.  If the switch is working properly then I would check the
adjustment of your idle air screw (on throttle body) and then vacuum lines.
Finally it could be your idle stabilizer unit.  I have these new for $25 each
(for 1.9l Vanagons only).  Hope this helps.

Thanks,
Ken Wilford
Van-Again
John 3:16
http://www.vanagain.com
Office: (856)-765-1583
Fax: (856)-327-2242