I've got an 1984 Lx with a 3.8 and I've been having the same problem for quite a while now. Mine has a really nasty studder/sputter at idle, and accelerating from nothing to about 60. Well, unless its at wide open throttle.
I've replaced the plugs, wires, rotor button, distributor cap, coil, fuel filter, EGR valve and its still there. I've searched for vacume leaks several times and found nothing. I know several people that have had 83-84 mustangs and thunderbirds with the 3.8 and they've said that they had the same problem. You're not alone.
If you figure it out let me know.