rough idle

Replace the ignition coil pack. On the 3.9 V6 it is located on the back valve cover. I have replaced it on my 3.9 V6 Freestar as well as on my Mercury Sable. They are not that hard to cahnge and Advanced auto parts sells one with a lifetime warranty.

Have your mass air flow sensor tested.

Before you get to worried stop by auto zone advance auto any parts store and get a can of mass air flow sensor cleaner if u don't see it just ask a clerk this solved this issue for a year b4 I had any problems with this again just be careful not to slam or bump anything to the metal shiny part that sits in center of air intake and it's only held on with 2 small screws just spray it down all over where it isn't plastic let air dry don't wipe off spray is made to evaporate replace sensor screws and if this the first time u have done this u should b good 10 min tops labor in 8 bucks or something for the spray