My idle was surging from about 600 rpm to about 950 rpm. I changed my PCV valve screen (filtre). Good bye idle surge. Don't see any mention of changing the screen in Chiltons or Haynes. If you haven't ever changed yours, try this. Get a rod about 10 inches long, 3/16 in. dia. with like a wood screw at the end and a handle at the other end. Pull the PCV valve out but leave the rubber grommet, the screen sits under the grommet. Twist the rod 2 or 3 turns into the screen (like a wine corkscrew) and pull out the grommet and screen together. Replace or clean the screen and hopefully your surge is gone. The way it looked, I think my screen had never been changed. My idle surge cleared up completely and has not returned with this simple and probably rarely performed maintenance procedure that I got from Mark Houlahan’s “Mustang 5.0 Projects”, lots of good how to’s in his book. Good luck.