Don I have heard about this overheating syndrome many times with Vanagons and
most of the time it is NOT headgaskets.  It is usually some obstruction in the
coolant system (mainly a bad radiator).  People have driven themselves crazy
trying to replace head gaskets, flush the coolant system, change thermostats,
etc to no avail.  So my opinion is that it is either a bad radiator (I sell
these new for $175) or the two front radiator hoses are switched (I have heard
of this causing overheating too).
Hope this helps you.
Ken Wilford
Van-Again
John 3:16