How are the O2 sensors?
Have you done the Salt n Pepper shaker mod yet?
Ford had a TSB out on the 87 - 92 Salt and Pepper shakers. Mid year 92 and the 93 models were corrected. But from what I've read the contacts inside dont always meet properly and when they age it gets worse. The mod is to pull them apart, clean them up, tweak the top of each pin so that it swells the end to make it larger. Then put some dielectric grease in it and put it back together. The symptoms are erratic idle and surging when they go bad.

When you reset the idle,.. did you do it with the IAC unplugged and set the TB screw as low as it would go before the car stalled out?
I just went through some of this with my son's car. I'm no expert with EFI. I want to be up front with that. I'm just going by what I've read and the results that I got by tweaking on it. I finally got the car to where it would idle and run good. Still wasnt perfect. But it was much better then when I first started.

Also,.. just out of curiosity.
Can you get a fuel pressure gauge and see if the fuel pressure is dropping at all?
Because it could be possible the pump could on its way out,.. or the pickup tube or line could have a clog or the filter may have a clog.

Also,.. what condition are your plugs in? When I saw your car last you and your buddy's cars both seemed to have a good bit of blow by in them. Fouled plugs can create different problems as well.
Pull 3 or 4 plugs and see how they are doing. And I know this may sound dumb,.. but have you double and triple checked for vacuum leaks? Even the most minute leak can create problems. Also,.. any kind of exhaust leak before the O2's will send a bank lean code to the computer causing the motor to run pig rich as it tries to correct a problem that isnt there. I went through this with my 96 and couldnt understand wtf was going on. Then found a small tick where the gasket was leaking on cylinder #8. It was so easy to over look it because the exhaust itself was pretty loud and the tick was very minor. Replaced the exhaust gaskets and viola. No more bank lean code.

Just some ideas to look into.

This is why I honestly dont like EFI. Its nice when its working right. But you'll end up searching for hours, days or weeks even trying to figure out a problem. And the thing with EFI,.. symptoms arent always what they appear. Like the bank lean code. Motor wasnt lean,.. it was pig rich. Plugs dont lie. Yet thats the code I had. And a simple exhaust gasket fixed the problem. With a Carb its right there in front of you,.. spark and fuel. Very simple to diagnose and correct. Not saying one or the other is better. Just my reasoning to prefer a carb over EFI when it comes to modified engines.

Hurst