issue

when throttled bout 1/4 or less i get what seems to be a misfire. sometimes when im at a light the rpms will drop from 900 to bout 400 real quick and back up. once in awhile it will acctually jump up a few hun. rpms.(its an auto so thats not good) i did get a code for a mass....i replaced it and its still there. any ideas? yes im gonna go for the plugs and wires tomm. but i was just wondering if anyone else may have had this problem and how they remidied it.