Resistor vs non-resistor spark plugs

faulty intake manifold maybe?

No, you've got no idea whether that is the correct setting. Your car manual should have the correct settings. Failing that, if you google "Spark plugs gap setting 1991 ford mustang" you should get an answer something akin to:-

( .054 inch ) - according to www . motorcraft . com
Read more: http://wiki.answers.com/Q/What_is_the_spark_plug_gap_on_a_1991_mustang_gt#ixzz1UjTk6dqh

just replace one sparkplug and wire at a time. this way all will be ok!! GOOD DAY!!

Hi :) Here is the 2002 Ford Mustang GT wiring diagrams for sparkplug wires.
4.6 liter SOHC V-8 VIN "X"

yes, just a spark plug socket and a spark plug gap tool

I don`t know if they make a non resister plug to fit that engine,but if they do,providing it is a plattium plug,only thing the resister plug is for,is radio noise,resister resist the noise from the radio.If you put just any plug,besides what it calls for,oem,it can,and most likely will,use lots of fuel,run sluggish,and set codes in the computer,besides give you bad gas mileage.

the spark plugs and all filters should be changed.

It is not possible to use Nologi non resistor silver spark plugs together with Nologi Hot wires on my Tiburon 2003. I guess it goes for all Tiburon from 2002 to 2006 with the 1,6 liter DOHC Alpha Engine. But I am still working on it and if I get anything to solve this problem I will be back to tell.

Very small help:
> ,(& what is a “ballast” resistor? Is it different from a regular resistor?).

Ballast resistors are special; they have what is known as a 'positive temperature coefficient' meaning that they increase in resistance with temperature.
This provides a very hot spark to start, then a reduction in current as the resistor warms up.
This prevented points and coils from frying from unnecessarily high current.
Early electronic ignition systems still used them, I don't think any modern car has them now.

If you have a larger public library, it might be worth a look on line (if available) or a visit to search for a manual; have found some classics there.
If you find it there - copy it, then RETURN IT! :-)