One problem that I noticed in the past is that only the side of the radiator
where
the hoses were got hot. The problem is that both the inlet and the outlet
from the 
radiator are on that side. Sometimes a crack will occur in the wall of the
radiator
inlet section and what ends up happening is the water goes into the radiator
through
the inlet and comes right back out. Since the fan thermostat is on the other
side of the radiator
it never gets an indication to turn on. 
The only way I know to verify this is to remove both hoses from the radiator
and look up the 
tube for an obvoius crack. Otherwise you could get a long screwdriver and
run it along 
the inside wall to see if you can feel it.
I had this problem on a Vanagon that I worked on and it solved the overheating.

Hope this helps.