I have a 2001 Eclipse Spyder and I've seen other sites where this is a common problem. Heating works fine except when idling at a stop. I read a suggestion (and just tried it) of taking off cap and warming it up and squeezing upper hose to try to rid the system of air. Didn't really see bubbles but I did top off the radiator and let it run. Hoses hot as was thermostat body so I assume it opens just fine. Have not yet all combinations of settings yet to compare defrost with floor with top or left and right. Had new radiator installed in 2012 and hoses; need to check if new thermostat would have been put in. Any thoughts welcome.
Thanks

 

So what does it do idling at a stop? It stops blowing air at all or the air gets cold as your sitting there?

 

Air still blows, gets cooler then warms up while driving

 

Sounds like partially plugged heater core. Is one heater hose considerably cooler than the other? If so Bingo! Clogged heater core. To fix remove both heater hoses at easiest location and put garden hose on cold heater hose and put other hose in bucket. Turn on hose and flush out crud/ buildup out of heater core. Reconnect hoses and fill with coolant. Quick fix compared to taking dash out

 

Also found later (but hasn't happened for many weeks) that the AC also experienced not blowing cold air briefly.

Two other ideas have been thermostat or a belt(s) needed tightening?