Engine idles too low when warmed up

I live in Las Vegas and this generally only happens in the summer when it is very hot. I start the engine in the morning it idles high at 1,000 rpm's and then settles down to about six or 700. After I drive in a while, park it, then get back in and started up it wants to idle at around four 500 rpm's and if you put it under a load such is turn the steering wheel it wants to stall out and stumble a bit before you get underway. After you are underway it is no problem at all. Generally this is a problem in the summer heat. Not so much in the winter.