I have a double vanity in which the sink on the left drains very slowly when water is first run. If I let the water drain out then run the water again immediately the sink drains normally. The popup, tailpiece and trap are all clear and operate normally; the sink on the right does not exhibit this problem. This is unlikely to be a blockage since the problem has been present since the house was first built; also, if there were a blockage I don't think it would drain slowly one minute and then freely when the water is run a second time.
Any ideas?