I have a similar problem with my 85.
 The idle jumps up to like 2000 or so then back down to 800 or 900..I went
through the archives to see if it was a common problem and I guess its
normal for the fuel injection system to do this.  I searched for this a
while ago so im not exactly sure but I think at idle gas is shot to the
engine then when a certain idle is reached the fuel is shut off till idle
gets to a low point then fuel is injected again, till a the idle goes back
up..thats were the intermittent idle comes from. again it was a while ago
when I searched.  its in the archives. that much I know.
Not sure if this is whats happening to your van.
Chad
72 westy
85 Vanagon   for sale