I think I'm experiencing the same issue and I was advised to clean the MAF (as I have replaced the IAC recently) What's the correct procedure and what would I use?
Another thing I just thought of is if you have a busted cat, it could be clogging things making it idle funky, and sound weird. That could be intermittent also. Do you have cats? if so.. check them.
From his original description it sounds like the cat to me. Sounds like the brick in the cat broke and is floating, thus restricting exhaust flow. Jack your car up and smack the cats. See if you hear anything jiggling when you do. If so, its new cat time. Thats the most likely guess as to what the problem is IMO.