[s-cars] overheating dilemma

[Tony Guttmann]

At the risk of abusing the list with a non-Scar question, my daughter's
Audi 80 2.6litre V6 quattro is overheating, and we don't know why! The
radiator, the water pump and the thermostat have all been removed and
checked. The head gasket has been tested two different ways, and seems to 
be sound. No water in the oil. No leaks evident in the hoses, cooling system holds
pressure. Fan sensor has been replaced, and fans seem to kick in appropriately.
Ambient temperatures here (Australia) are no more than 30C at present (86F), 
and the situation manifests itself in stop-start city driving.

My mechanic is running out of ideas! Any inspiration from the collective
brotherhood (naturally, this is used ultra-collectively to include any sisters
out there)?

Many thanks in advance for any suggetions!

Tony Guttmann