A/C problem:
Could be the A/C relay is stuck, have it check.

Overheating at traffic or idling:
Is the fan coming on? At your home, let idle your car for at least 10 minutes or so w/ the hood open and watch the temperature gauge as rises to almost overheating, if the fan doesnt kick in that is your problem. Could it be the motor, relay or sensor. If you have a secondary fan for A/C check that thing too. You have to replace your thermostat also, replacing it will not harm your engine and they are cheap.