If its only overheating when sitting still then it's most likely that your fan clutch has worn out. When it fails the fan spins freely and doesn't pull air in to cool off the engine. A good way to test this is take a small newspaper or flyer and roll it up. Then with the car running slowly insert the newspaper into the spinning fan blades. Do this slowly and carefully so you don't snap a fan blade. If the paper stops the fan then your fan clutch needs to be replaced.