2005 Chevy Impala

My 2005 impala had the same problem. Here's the answer. The heater core is plugging up. When you sit idling, the fluid movement is slow, therefore the junk settles in the heater core, still allowing enough fluid to get through to heat. When you drive it, the fluid movement is much faster, therefore sending the junk into the heater core, plugging it up. To check for this problem: Get engine temp up to normal, feel both heater hoses. If one is hot and the other one is cooler, fluid is not passing through. Take off both heater hoses at the top of the engine, blow out fluid with low pressure.(5 or 10 lbs.) Fill both hoses with the right radiator flush. (Some engines are aluminum) Let stand for 4 hours. Flush with new antifreeze. (I used a garden sprayer) Stick the sprayer end in the heater hose and tape tight. My vent temp WAS 100 degrees. After fix was 152!