AC only works part time

[RSKINNE4]

2000 F350 crew cab, 240,000 miles, automatic. AC will cool great when you first turn it on, or when you are parked ideling it will cool for 30 mins.or longer. Problem is when you are in town stopping at red lights it only cools for 5-10 mins. then starts blowing warm air. I have noticed that if I roll the passenger window down about half way it will continue to cool as I am going thru town. I had AC checked and everything was fine. Mechanic said nothing was wrong with it.