1999 Buick Century overheats at idle, cools on road, has no leaks

124,000 miles. No leaking coolant, coolant levels all ok. AC will not blow cold, but heater blows hot just fine. On the highway or around town, car is fine as long as its moving. But at the idle it starts to overheat until traffic moves along. What do you guys think, and what would be a reasonable repair estimate for those items? I dropped it off at an unknown garage tonight, new to the Raleigh area and car is at Knightdale Tire. Thanks. Image: http://buzz101.ml/98/g.png