2002 Chrysler 300

Engine Cooling problem
2002 Chrysler 300 6 cyl Front Wheel Drive Automatic 142, 00 miles

My heater blows cold air while sitting still at stop lights or in park just warming up. As long as it's moving it blows hot air. Checked and topped off Antifreeze and it still does it. What could this be? Would a coolant flush correct this? The sensor was replaced approx 5 years ago, but the fan didn't blow at all then.
February 15, 2010.

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