I have the same engine stalling problem starting about a month back. The engine starts up with a single try. Then after about 20 seconds of idling, the RPMs would slowly drop and around 200 RPMs it sputters and dies or kicks back up high to 2000 RPMS. Restarting is 100% successful and so are other times throughout the day.
Then when I come to a full stop or close to a full stop, the engine dies very smoothly and unnoticeably (like shutting of the key). It died twice yesterday in the rain. Both times I could start it up in 1 try.
Two weeks back I had a complete tune-up done (sparks, coil pack, fuel filter, PCV valve, etc.) along with a oil change. Plus I put in a new IAC. Also pulled the #4 and #11 fuses to clear the computer. I thought it had cured it but it's back yesterday. So far I've detached and reattached the battery securely. I read from an old post to try using some ISO-HEET and fuel injector cleaner solution. I bought some and just have to put them in.