Mist coming from AC duct
 

I have a 1995 Lebaron GTC conv. V6 auto.When I first turn the ac on it1s fine,then about 10 minutes later a mist starts to come from the ac ducts.This lasts about 15 to20 minutes.This does not effect the ac, it stays cold.then it stops.Anyone have an idea what this is caused by,and how to fix it? Thanks