1999 and 2000 sunfires overheating
 

I have two Sunfires, and both have developed same problem now that summer heat is here. If running at highway speed, A/C works great on both, and temp guages stay at normal. If idling in traffic, both overheat and A/C blows warm air. Both OK on antifreeze. What problem should i look for, and is this common for Sunfires, or am I just lucky?