I would check your engine grounds first.  The grouding strap between the
engine and the firewall on the driver's side (US) should show zero resistance
when checked with an ohm meter.  The connections should be tight and clean.
After looking these over, I would check your Temp I and Temp II readings.
Usually if there is a problem here you can easily detect it with the
Digitool.  When the van is stone cold in the morning, the Temp I and Temp II
readings should be the same.  If they are different then you should reference
the chart in the Bentley to see which of the two is within limits and which
is out.  Temp I is built into the AFM and so if it is bad you must replace
the AFM.
Check your intake and exhaust for restrictions.  I know a list member posted
a similar problem only to find that it was a collapsing intake air tube from
the air box to the intake up at the opening on the body (passenger side vent
at head level).  Un hook this air hose and check your air box for any mouse
nests, etc.
If this is OK then be sure that your catalytic convertor is not plugged.  It
could have died restricting itself or the muffler.
If all of these things check out then go over your tune up parts (cap, rotor,
wires, etc) and fuel filter.  Be sure all of these items are in good shape.
If you have someone local with another 86-91 Vanagon it would be a good
proceedure to try swapping some known good parts from their van onto yours.
ECU, AFM, ISCU, etc.  That way you can quickly troubleshoot these items
without spending a bunch of money "throwing parts" at the problem.
I hope this helps.

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