I don't know about other states, but in Calif. the gasoline is oxygenated from, I believe, November, through February. I've been told that oxygenated gasoline can cause some "vapor lock" type problems if the weather gets too warm.
My car has been idling roughly off and on for a while. Today I put some gasoline in the car, which I believe is now probably non-oxygenated, and so far the idle is very smooth. Could the gasoline have been causing or contributing to the rough idle?