Hi David,
My 84 did the same thing...it was the lower speedo cable (dealer $340, maybe
available at one of our list vendors for less).  Evidently, the twisting
force(from lack of lubrication) causes the "spring like" cable to lose its
tension.
Try that and let us know:>)
Karl Batzler
84 Vanagon
191,000 mi. and counting!!!
P.S.:Maybe you can try lubing the cable with some graphite and see if this
does the trick.

-----Original Message-----
From: David-M <dmc@CYBURBAN.COM>
To: vanagon@VANAGON.COM <vanagon@VANAGON.COM>
Date: Tuesday, October 20, 1998 8:26 AM
Subject: Speedo 'jigger'

>My speedo 'jiggers' at around 40mph, ie. it wavers up and down, from say
>37 to 43 rapidly and makes a clicking sound.
>This goes away at lower and higher speeds.
>Is this just something lose at the back or something more serious.
>