My car runs great except when stopped at stop.

My car runs great except when stopped at stop signs or lights. Sometimes (not always) it begins to kick and sputter. The RPMs drop from about 800 down almost to a stall, then jump back up again, over and over again. It has stalled once or twice. If I shift into park and take my foot off the brake (it is an automatic) it seems to improve somewhat. No mechanic I have talked to yet, has been able to tell me what is wrong.