Of course you should check all of the normal stuff first:
Filters, distributor cap and rotor, plugs, and then go through the tests of
the ignition system that Bentley discribes.  If you have another 86-91
Vanagon available I would then try swapping in the ECU.  This could be the
problem as the ECU has the ignition module integrated into it in the later
vans.  These go bad more than I would like to think about, but since I have
spares sitting around for testing purposes, I can always narrow down the
problem in a matter of minutes.  Hope this helps.

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