Hi Creg,
Don't use HTML code next time!!!

You quote:
--The vehicle gets hot enough to boil the coolant-
How do you know that for sure, the VW engine run's quite hot!!!

What is your normal operating temperature? (needle on the dash on freeway)

If ok; maybe you don't have enough pressure from the pump at idle, but I
doubt that, my pump was finish!!!! and I still had good cooling flow!
Unless the engine core surface (where the pump is attach) is not even,
pitted or damage.

Feel the hose on the front upper part of the thermostat housing (the big
one that goes to one of the big rigid cooling tube) when the engine is very
hot (and fan working) and compare the heat of this one to the one on the
right (Bypass form the distributor to thermostat), , If very hot, that tell
you if the thermo is opening and mean that coolant is flowing from rad too
housing. (good news) BUT; coolant can flow but is it flowing enough!!!! If
the front rad is partially clogged, it will allow SOME coolant to go true
but not enough!!!  I have never seen a clogged flexible line in my life!

Does your temp gauge sender works adequately. (The one attach to the temp
gauge in the front located on the thermostat housing)

Most thermostat fail in the open position, so if it's failing you shouldn't
have a heating problem. How old is the thermostat? No need to completely
bleed the van when the thermo is change (just open the thermostat housing
bleeder for a few second with the expansion cap remove-engine cold)

Here is my suggestion, change thermostat first. Put a lower temperature
one, available everywhere. (80deg)

If you still have the problem you will have to remove the front rad and
flush this one whit clean water or put a new one.

The way I see your problem is that you don't have enough flow at idle, pump
could be the problem also or simply the mating surfaces.

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