I would wager it's got nothign to do with the AC. If the system works well on the open road, but not in traffic, it tells me that the system is operating properly, but it's not effectively radiating that heat to the atmosphere through the condenser coil on the front of the engine.
I suspect that the engine cooling fan is not moving as much air at idle as it should, therefore, the condenser is not being adequately cooled (either that or it's packed full of bugs and dirt and debris.... check that first).
There are two options: You could try replacing the fan clutch (between the water pump and the fan itself) or, (and this would be my suggestion) replace the mechanical fan with an electric fan. The electric fan will move more air at idle than the stock fan which will keep the engine cooler. Additionally, if wired properly, it will cycle every time the AC compressor cycles, again, moving more air across the condenser coil at low speeds.
Those would be my suggestions.
-Joe