gmcc

I have read every thread I can find and posted a few times about the heating system in my 07 525xi with 18k miles
The dealer has tried but I am still not satisified that it is working correctly.
So far the dealer has replaced the thermostat and water inlet (thermostat stuck open), replaced the heater control module, reprogramed the heater doors and replaced the radiator (had an internal leak).
I just returned from my first long cold weather drive after the repairs and I have some questions.
While driving at highways speed (70 to 75 mph), in 25 deg F outside temp, for an hour I had to keep the the inside temperature set to about 74 and constantly felt cool air on my feet. As soon as I exited to a secondary road and slowed down to 35 mph, the there was pleanty of hot air coming out of the the floor vent and the dash vents. So much so that I had to turn the temperature down to about 64 to stop the all the hot air.

I was wondering if anyone else has any similar experiences. In my other cars if I set the temp to 74 it maintains that temp no matter what speed I drive.
Gil