I didnt know what to name this thread but Ive noticed for a long time that when I have my truck running at an idle just sitting there with my lights on and then i roll up/down the window that my headlights dim for the duration of the rolling up/down. But recently, in warm or cool temps, even when i shift from drive to park or any other gear I notice that my headlights dim plus the rpm's go down to 600 or so rpm. Just this last spring I bought a brand new motorcraft battery so I dont think thats the problem, could it be my alternator?

 

Mine Idles at 600 rpm, I only notice the headlights dim when ABS kicks on. I had my alternator bench tested for free... there was a squeaking problem but it wasn't the alternator which passed flawlessly.
When the truck is off I get 11v, when it is running it is at 13v but that is at startup which is around 1100 rpm. In the tuning software I can see that if the system does not have enough voltage it will bump up the rpm to charge the battery....
However my truck would never idle at 600 with the stock tune.

Maybe your IAC valve is bad?