I have a 2004 mercury marquere. When I drive about 45 to 50 miles per hour it puts out cold air. when i get on the interstate and go about 60 or more miles per hour it puts out hot air. as soon as i get off the interstate and slow down to about 50 miles per hour or less it starts putting out cold air. i have had it the shop 2 times and they cannot find anything wrong with the air conditioner.
This is a WAG…
I wonder if the air at that speed is forcing the duct to open. What happens if you put it on recirculate???