If it doesn't work in stop and go traffic, it could be that your cooling fan on the radiator isn't working properly. If the fan doesn't work correctly, you won't have air flow through the radiator or the ac unit(condensor??) in front of the radiator. With no air flow, the condensor (or whatever it is called) heats up, and the high pressure keeps the ac from working. Or something like that.

95 Contour Zetec, ATX (wrecked, sitting in the barn) 95 Mystique Zetec, MTX 2000 Taurus SEL 3.0L Duratec 1994 Crown Victoria LX 4.6 V8 1993 Chevy Silverado 4.3 V6