>  I think it is just a poorly designed system that works
marginally.

In the ten years I have owned my Transporter I can only
recall the high speed fan coming on a few times. I believe
the radiator, when working efficiently, to be over
engineered. In fact it is cheerfully retained by the lunatic
fringe after a V8 transplant.
My point is, maybe you should consider taking your radiator
out and having it professionally blasted out. Personally, I
do not recommend the do-it-yourself system flush.

Ken Lewis
86 Transporter Crew Cab, 60 T-5 Coupe
http://neksiwel.20m.com/   O2 sensor monitor gadget, my
cars, Vanagon Tech
http://EB03.20m.com/  Everybus 2003 pictures