I would check the thermostat.  It could be hanging open and causing the van
to run much cooler than it is supposed to.  This can cause the symptoms that
you are experiencing and many times an overheat will kill the thermostat
(lets the wax inside melt out).
The other thing I would check is the groundstrap from the head on the
driver's side to the fire wall just under the coil.  Should be there (some
times it is missing) and should be in good shape and connections need to be
clean and tight.
Let me know what you find.

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