Hey,it sounds like you have my car.I have had the same problems,replaced gaskets,no vaccuum lines hooked up to the EGR.
My car was initally hunting real bad until it stalled out,now it will idle at 2000 and you cant do anything to change it.I have been told multiple things,from i need new injectors to a tune.Well I have decided that I am going to build a carbed 351w b/c I am tired of dealing with it.I was also told by a guy that I work with that the MAP sensor can cause that same problem,just havent replaced that part yet.
I hope that you find the answer to your problem and good luck.If I find the answer I will post,but I have pretty much said forget it and quit working on it,I just drive it and deal with a high idle,if it blows it up,then I did the car a favor and I have an excuse for changing to a 351w anyways.Let me know how this turns out though.
1997 Mustang GT Convertible w/ xpipe,flowmasters and turndowns,sounds good.
1990 Mustang coupe
Afr 185's,Lunati cam,RPM Preformer,650 Quickfuel,and lots of other goodies