check your other 3 plug wires.
how are the coils....i was not sure how to check mine but it had some cracks and i replaced them....made a small difference.

i realized afterwards that part of my engine feeling rough was the tranny mount which was bad......it obviosly wont solve a idle or backfire problem but can confuse things.
check my old thread with an answer from arthur dalton on fuel pressures.

hope this helps