under dashboard, drivers side

What is your question?

I have a 2002 Ford Escape with 3.0 Liter engine. The engine has just been replaced tranny rebuilt 1.5 years ago, AC rebuilt last year. Now when I am driving in town with the traffic lights the ac starts blowing warm air and engine sounds like it is about to die.(running very rough) I can push the pedal to the floor and it sputters and coughs a little and eventually is will run normal, and the AC goes back to blowing cold air. If I am on the freeway it runs like a charm as long as I keep the RPMs higher than 3500. but if I start slowing it blows warm air unless I get the RPMs back up. Does anyone know what is wrong with my car?