Probably a dirty Idle Air Bypass valve (located on the right side of the TB). Disconnect your battery, then disconnect the IAB plug and remove the 2 bolts holding it to the throttle body, then remove the 2 little screws that connect the valve. Using carb cleaner, soak the inside of it and clean off the rod so it moves freely, then air dry and reconnect everything...but first, use that carb cleaner spray (and a lot of it) on the TB and wipe it off, especially around the flap. Finally, reconnect the battery. This should help your idle straighten out. I do this as regular maintainance every spring and fall, along with a can of Ford fuel injector cleaner, and have never had a idle problem in 94,000 miles on my '90 LX 5.0. I hope it works for you.
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