I have a 90 Nissan Pickup. It sputters and backfires really bad.

What is your question?

It sputters and backfires really bad. I've changed out spark plugs and wires, coolant sensor, o2 sensor, IAC valve, EGR valve, MAF, distributor and rotor caps, catalytic converter, fuel pump and filter, seals on fuel injectors, timing chain, vacuum lines. I done everything I can think of and its still sputtering and backfiring. Please help me, what am I missing? What seems to make the problem better or worse? no change hot or cold How long have you had this problem? for a couple months