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.