1990 Chevy Silverado Staslls, misfires, backfires

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires

Speedycato

You practically mentioned everything that I've thought of-in your PM. Okay lets try the knock sensor could be retarding the ignition timing and double check the fuel pressure with and w/o the fuel pressure regulator-

Could be valves/camshaft problem

You need to follow up on what I've suggested earlier this problem can be a nightmare to remember its not a simple fix