Fuel Pressure Regulator?

86GT-
If you are getting low voltage readings from your O2 sensors...then you'll probably need to replace them. That's how the sensors work. If the voltage rises or drops below the predetermined O2 settings...then it sends back a low/high (usually low) voltage signal to the EEC-4. How many miles are on your oxygen sensors? Replacing your O2 sensors might be a good step in figuring out your problem.

Stuart-
If your car is feeling sluggish, and none of those things you've replaced or tinkered with have helped...than try pulling out your injectors and getting them cleaned. Or throw in some new ones. My wife used to have a '93 GT (we both had Mustangs for a few years...pretty cool) that got REAL SLOW after some miles started racking up on the motor. I tried everything...plugs, wires, cap, rotor, fuel filter, injector cleaner, etc...nothing helped. I eventually installed my old stock 19lb. injectors (they only had 60,000 miles on them) that I was no longer using (have 24s now), into my wife's Mustang and...walla! Fixed! It felt like a new motor!!