Radiator should be one of the last thing to blame on a Vanagon! Except for
that stupid bleeder screw, is it loosing pressure there?

Radiator Wisdom: They don't clog easily, that story from my grand mother.
But they do loose pressure, that is a fact, when they do loose pressure you
could se an increase in temp at freeway speed, where the engine is producing
more energy and needs efficient coolin. (my racing experience)

You can flush your rad with city water!!! afterward, let this one dry in the
sun , if the flow is nice a the other end of the rad this one is working
fine, look for something else.

Heating problems/cause in order (if no external leak):
-Water pump (main problem)
-thermostat (no they don't always break fully open, believe me)
-lost of pressure
-gain of pressure
-front radiator
-air pocket
-clog passage (not a real problem on Vanagon)
-old coolant (put some brand name fresh coolant and pleaseeeeee, forget that
phosphate free crap!!!
-your van is possess by an evil spirit (and was built on planet Mars by
drunk VW engineer)

check coolant pressure and coolant quality, if you are perfectly sure that
the pump is doing is job... Are you sure?
http://www3.sympatico.ca/huotx/pressure_test.htm
http://pages.infinit.net/engine/cooling_system_flow.htm
http://www3.sympatico.ca/huotx/front_radiator.htm

Cheers, Ben
http://www3.sympatico.ca/huotx/engine1.htm