2006 Ford F150 Odd air behavior

Having trouble with the air in my F150. When the truck is started, the air does not blow cold. I can let it idle for 15 minutes, and it does not blow cold at all. Once I start moving, the system will gradually blow cooler air. If I stop, say at a red light, the system stops cooling again. This has happened since the truck was new. I've had it for warranty service 6 times; each time the dealer says it's working like it should, sometimes after topping off the refrigerant. After they top it off, it seems slightly better for a month or so, then I'm right back where I started.

Thanks for any ideas...