I would try to clean out your idle stabilizer valve (on the top of the motor
in the center) with some mineral spirits (brake cleaner).  If that doesn't
fix it then it is probably your idle stabilizer (behind passenger side tail
light on 86 and newer vans).  Best way to check this is to swap in a known
good unit.  I have both the idle stabilizer and the idle air valve on my web
site if you need them.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242