if you have a battery charger, hook it up to a low amp trickle charge for a couple of hours. Sometimes a battery with a weak charge will cause the idle to drop when slowing down, and I know you run subwoofers, so your battery charge might be a little. weak, see if that doesnt help it. An IAC valve on the duratec is an easy fix either way, just the part isnt the cheapest thing in the world.
