[84LX89GT]

I would look at the fuel system first, if everything checks out, the next thing i would check out is the ignition system. You can check out the resistance of the wires with an ohmmeter (specification can be found in chilton's books or ford manual), also check what the spark plugs look like and look at the condition of the contacts on the rotor and inside the distributor cap, and if you still don't see anything check the compression of the engine to see if any cylinders have less compression than the others. Also checking the timing would be advisable to eliminate that as a possibility.
It's all just a matter of narrowing things down. Also make sure to write everything you do down and your results so you can refer to it when looking stuff up or remembering your steps.

If your fuel pressure is low, try replacing the fuel filter first, then disconnect the fuel pressure regulator vacuum hose while your car is running to see if pressure goes way up. If it doesn't go up, the regulator is faulty because even with a weak pump, pressure should go up - the volume just wouldn't be there with a bad pump. Some fuel pressure regulators stock on mustangs are adjustable on older models, so you may be able to tinker around with that.
If fuel pressure is way too high, the regulator is bad.
It's all a matter of getting info from tests and interpreting it.