Larry,
   I applaud you on your concise and logical trouble
shooting methodology. Two possible additions:
1> Jumper around on the radiator fan switch to see if high
speed works.
2> Run both heaters full blast and try to duplicate the
boil-over condition. If you can not I would suspect the
radiator. If it does over heat i would suspect the water
pump.
Good Luck.

      Ken Lewis
 86 Transporter Crew Cab, 60 T-5 Coupe
 http://neksiwel.20m.com/   O2  sensor monitor gadget, my
cars, Vanagon Tech
 http://EB03.20m.com/  Everybus 2003  pictures