Sounds like it might be the famous Idle Stabilizer Unit problem.

Bus Depot has them at a very fair price

Dane Tessler
http://members.aol.com/Dane714/westfalia.html
-----Original Message-----
From: Jarod Bishop <jbishop01@FOXINTERNET.NET>
To: vanagon@VANAGON.COM <vanagon@VANAGON.COM>
Date: Thursday, November 19, 1998 4:47 PM
Subject: slight surge in idle speed

>I have an '87 Syncro that seems to go through a cycle when idling while
>stopped.  It drops down every 3 seconds or so, so much that it shakes the
>van a little for about a second and then it's back to normal.
>
>Just wondering if this is normal or if I should check it out.
>
>Thanks
>