My car did something similiar. To test it, I turned the car on and let her idle, she went from 500 to 2,000, then back down. While it went down to 500, we sprayed carb. cleaner on the upper intake manifold gasket. While we sprayed the upper intake manifold gasket the idle went back to normal. You could hear the idle smooth out.
Now I am in search on the intake gaskets in town. I think I found them. It's cheaper tp buy them from Bill Jenkins, but I don't have time to wait for them to be shipped. I am a little sick of sitting at sit lights holding my foot on the peddle hoping she doesn't kill.
I called many Ford dealerships this morning and they all said the intake gaskets were really popular since the temp dropped. We are at 0 degrees right now!! Brrrrrrrrrrrrr....
Good luck!
We have been trying to figure out this problem since last week. And finally!!