My first guess is the aux air valve. They should be fully closed before engine thermostatic temp. If they are still somewhat open at idle engine temp they can close further after the higher heat of the freeway travel. Set idle speed to 850 after its real hot and see if its too much other times.
Another alternative is the mixture is lean and the engine temp sensor is still changing in that temp range. The resistance of the eng temp sensor is logarithmic in its change. Very high when cold above 1500ohms. At idle it is down to 150 - 300 ohms. VW and Porsche used a 200ohm resistor in series to nulify the swings in resistance in the last 20 degrees of engine temp. Adding the resister makes it significantly richer at warm setting (but stable) but makes very little difference in the cold mixture due to the relative size of the two different resistance totals. If the main mixture is now too rich use my basic D-jet mixture correction method at the manifold pressure sensor.
Bosch Master, ASE Master, L1
33 years MB technician