Sounds to me like your refrigerant pressure may be a little low. When you're at speed the compressor keeps the pressure up, but when you're idling the pressure goes low and the system pressure switch cuts off the compressor to prevent damage. Once you take off the pressure comes up, and it's cooling again. I'd have the refrigerant pressure checked.

If it's not refrigerant or receiver/dryer then the next thing will be to talk about your fan, clutch and shroud. I would not put a pusher fan on, as it will restrict air flow for your mechanical fan all the time, especially going down the highway. You may introduce a cooling problem with a pusher fan.