99 Regal Runs rough in warmer weather

I have Regal with a non supercharged 3.8L. In cooler weather it runs like a top. When it gets above 70-75, its another story.

If I drive the car until its up to temp. shut it off (eg. go into a store) and come out to restart it, it starts up fine, but if you watch the tachometer, the idle is very unstable. If you rev it a little to get it to clear up the entire car will shake quite hard. Its obiously not running on all cylinders. The Service Engine Soon light is NOT on. As fast as it comes, it goes right away.