1987 Mercury Topaz Low Idle/Rough Idle

I have had this problem for the past 3 or 4 years! If the dash tach is to be believed, it idles at approx. 500-550 rpm (which is low). The idle is rough and sometimes feels like it will stall (but usually does not). I have replaced the plugs, wires, car, rotor, distributor, TFI, MAP sensor, fuel filter along with just about every sensor and vacuum line I could find. Also replaced the main computer and fuel/computer relays. I had all these parts in stock and did the work myself so I though why not? The strange thing is when going from a stop light/sign, the car picks up immediately and has great power (for a small 4 cyl). Except at idle, it runs like a new car! By the way, also replaced the IAC, TPS and the one injector it has. All Motorcraft parts. Have not run a fuel pressure or compression test because of how well it runs outside of idle. Thoughts? Thanks. Bobby.