overheating 4000q

[George Nimmer]

I have a puzzling problem with my '85 4000s quattro overheating.  It has
156k miles on it.

The overheating only occurs when the outside ambient temperature is over
90 degrees Fahrenheit, and only when traveling for 15 minutes or more at
greater than 70 mph.  When this happens, the water temp needle slowly
climbs to almost the top of the gauge, and very shortly there after, the
oil temp climbs until the little red LED at the top of the bar lights
up.

Again, it only overheats when driving for 15 mins or more, at greater
than 70mph, when outside temps are 90+.  It does this whether or not the
A/C is on.

It runs cool as a cucumber when the A/C is on high and I'm stuck in
traffic, driving in the city, or crusing at only 60mph, even when
outside temps are 100+ degrees.

I have replaced the water pump, thermostat, coolant expansion tank, all
coolant hoses, the radiator, radiator fan, water temp sender, and
flushed the coolant more than a few times.  I'm running a 50/50 mix of
distilled water and G12, and there are no leaks in the system.  In
addition, I had the coolant system pressure tested at the local VW/audi
independant shop, and had them ensure there were no air bubbles stuck in
the system.

What other components could I possible check???

The engine starts and runs marvelously, revs cleanly and smoothly all
the way to redline.  it is meticulously maintained, and I've taken
several recent ~500 mile drives without any problems whatsoever (with
the exception of the aforementioned overheating under certain conditions
situation).

Ideas?

Thanks.