Are you SURE the A/C is not working? These cars have a tendency to deveop a problem with the valves that control heater water. Mine will "stick" and give me 100% full on heat on the driver's side once in a while. The only way to stop it, is to shut the car off, then restart it - fixed. (Acts up when I turn the heat **** up fully, then when I turn it down, the valve seems to stick).

So, is the A/C really not working, or is the heat staying on?

If the heat is not staying on, and you can tell the pump is engaging, then either the condenser up ahead of the radiator is FULL of leaves, the fan isn't running (if it cools as you speed up, that would indicate the fan under the hood is not running when it should, but this would also imply overheating of the engine at times would also happen) or the system has leaked out.