I know this post is several months old, and you may have resolved your issue, but I thought I'd go ahead and post this info anyways. Maybe you, or someone else with idle problems, can benefit from it.
I have a 2004 Freestar. My problems span over the last three months. They range from the headlights randomly going out, to the radio turning off, lack of general power, misfires, and now idle problems.
I first thought I was dealing with a problem with the engine or battery not being properly grounded. I cleaned the ground wires and ensured a good connection. Once the misfiring began, I found that the coilpack was bad and replaced it. And the idle problems were very indicative of an idle air control valve issue.
After replacing the idle air control valve ($40), I saw no improvement in the idle issue. I did some research and learned that Ford published TDB 06-14-10 addressing issues with moisture getting into the powertrain control module (PCM). Apparently, moisture in the PCM can cause a variety of issues; including transmission problems, electrical problems, headlights turning off, engine overheating, and idle problems. The TDB is available for free on the internet.
I used the instructions at Ford Freestar Transmission Problems - Car Forums - Edmunds
to pull the PCM apart and found moisture on the inside of the 140 pin connectors. I did not find any within the PCM on the circuit board, however, rusty metal components around it indicate that I have had a moisture issue for some time. I used a qtip and a paper towel to wipe off the majority of the moisture. Then I carefully used a hair dryer (on low setting) to dry the connections out completely. It's a time consuming process but caution is necessary. After putting everything back together, The idle issues had vanished. The vehicle also seemed to perform with more power.
This weekend I intend to complete the instruction on the TDB to reduce moisture around the PCM. I also intend on using silicon to seal up a few areas where moisture still has access to the circuit board and connections.
Eliminating moisture from the PCM can reduce many performance issues with the Freestar and save owners a ton of money involved in replacing them.