palerider,

may sira yata yung auxiliary fan ng aircon.

similar din ang problem ko with my civic.. nagdo-drop yung idling upon turning on the a/c.. everything is normal habang tumatakbo yung sasakyan.. pero as soon as na-traffic, nagdo-drop yung idling and nawawala yung lamig.. kasi hindi umiikot yung aux fan ng aircon.. pero if you hit it with a screwdriver or something, aandar ulit siya. hehehe..

try observing your aux fan. if my guess is correct, have it replaced or repaired.

HTH