My 2002 Honda Accord over heats in mid to heavy traffic?
 

My Honda accord over heats when stuck in mid to heavy traffic This started not too long ago but now its very consistent i.e. everytime i am stuck in traffic it starts to over heat and the heat gauge in the dash rises all the way to the top which is critical. i have replaced the coolant in the are and the radiator is not leaking. The funny aspect of this is when the car starts to overheat and i turn on the air conditioner it stops over heating and the gauge goes down....this is weird and i can explain it....can someone help!!!