'86 4000quattro overheats on the highway

[selick@cs.bu.edu (Steven Selick)]

Ok. Here's the deal...

I've replaced the radiator core (professionally), fan switch,
thermostat, and the water pump, and the car still overheats on the
highway! There are 6 lines on the temp guage, and the fan comes on
halfway between the fourth & fifth, and goes off (when the car is
idling, or in slow traffic) when it gets down to the fourth. This seems
normal, but at highway speeds > 70mph, the car completely overheats ie
goes almost to the top line, and the red tep light on the dash comes on.
My mechanic is totally mistefied, and I've been without my car for 2
weeks (I bought it a month ago). Oh...it's got 177k miles on it, and he
said the timing belt is spotless, and there's nothing wrong with the way
it drives. The engine idles smoothly at around 800rpm, and just oozes
power. The ac just died by the way as well.

Please help my poor decrepid quattro!
-Steve
<selick@csa.bu.edu>