Same thing for me
I seem to have just about exactly the same situation. A/C works very well, plenty of cold air. Auto temp control works fine. Leave it set at about 76, cools when the car is hot, and heats when the car is cold.
After about 1 hour of driving now in the summer, many times the car will just switch to blowing hot hot hot air out of the vents, and it seems unclear if the side vents are blowing the same temp as the center ones.
I have to turn everything off, and sometimes that will fix it, or, if I let it sit overnight, it seems fine in the morning.
The fan speed initially doesn't change when the hot air starts, but after a few moments it does start to increase. I attributed this to the fact that the temp. sensor is now seeing that the interior air temp is increasing, and it puts on more fan to cool it down, but of course since it's blowing hot, it increases the problem. It doesn't know that it's blowing hot air. So to speak!
This says to me that the temp. sensing portion of the A/C is working OK.
On my commute home, it's a little over an hour, and it seems to happen pretty close to after about an hour of running. But not every day.
I'd really like to know what's up!
'95 E300D, 143K