The overheating is most likely your radiator. My van had the same symptoms
when I bought it. It was diagnosed by two "mechanics" as an inner head
gasket problem. Turned out the radiator, even though it seemed ok(would
pass fluid when off the van), was restricted enough to not allow the
coolant to
flow.

It is a large, long, upward coolant loop in our vans and it seems that when
resistance reaches a certain point it just stops flowing.  New radiator and
all was well.

I just replaced my friend's radiator a couple of months ago for the same
reason and it cured her problem as well.

The plastic connector you speak of also blew out on my van. I ended up
removing it and splicing the hoses with some home fashioned pieces. Worked
fine.

Last I saw, the Bus Depot had a really good deal on new radiators.

Good Luck,

Tim
85 Westy Tiico
83 Rabbit GTI 16V