Honestly could be a combo of things.... I used to have similar problems.. engine is apart now... but anyways... I found I had to change more than one thing to get back to normal... eg. I replaced what vacuum lines I could get at with a vacuum kit from mustangs unlimited.. different colors to choose... and that improved things a bit.. I also took off the IAC and cleaned it and as much inside the side of the throttle body as I could, I also took off my cold air intake and with one hand held open the tb plate and used a screw driver with a cloth on the end to get some of the sht out. Then checked the tps to make sure it was getting the 5v and that it was below 1v. Think I got to .8. And if you pull off the one vacuum line to the egr valve, it should either stall or sound like it is going to, and if it doesn't change than something is going on with that. I also replaced my tab and tad valves and solenoids with a kit from 50resto.. After all that everything seemed normal.. but I felt like each made an improvement! Good Luck! I would also check your engine error codes.
