Hello, I have a 1986 mustang and one day while driving my rpms wouldnt go down past 1250 rpms. After letting it sit for a few hours and starting it back up it would surge between 500 and 750 rpms. After a week or two the rpms would occasionally get stuck above 1000 rpms, so I swapped out the IAC. It fixed the rpms getting stuck and now its surging between 500 and 750 rpms when idling. Any ideas what the problem can be? Im at a lose right now.