Not as familiar with the diesel Vanagon setup as I am with other Vans and
Jetta/Golf, but I have had similar problems before and it was the coolant
level sender.  I forget if the diesel vans have one (I have pulled a couple
apart but didn't notice) but if they do, unplug it and look down into where
the two little metal contacts are that go into the plug.  Is there any
moisture in there?  If there is then the sender is bad and is causing your
problem.
Or is your coolant level just low?
Ken Wilford
Van-Again
John 3:16