i had mine to the dealer for the same thing.. stall or stumble when shifting from drive to park or reverse. they changed the IAC,, but it did not fix. i tried cleaning it before i took it in.. i started looking and searching for something and removed the intake duct and cleaned the intake butterfly valve the problem went away... i disconnected the duct, sprayed some WD40 on a rag and cleaned the flapper valve and intake area... worth a shot,, it fixed mine..
troy