I have a 2005 F-150 5.4L Truck with an Automatic Transmission. In drive, what should the trucks rpm be at. Mine is sitting right on 500 rpms and that seems low. When you slowly come to a stop some times the truck shudders and almost dies.

 

You have a fault with air bypass control.
The IAC may need cleaning.
The system is very good at holding idle under all conditions when there is no faults..