Hi my name is XXXXX XXXXX to just answers.
When did this concern start?
What injector spider does it have? the poppet style , or the updated injectors style?
Can you read the miss fire indicators to determine the cylinders that have the highest misfires? Or do you just have a code reader?
Here is what can cause the back fire. Condensation inside the distributor cap can cause a cross fire. If it backfires inside the intake, It can cause the injector tubes to melt and excessive fuel will flood the engine . (fuel smell)
The timing can be set and must be within 2 deg of 0tdc. it is set by reading the cam sensor offset, on a scan tool. But if it has been running fine and no check engine light up to this point, that is not you concern.
Check the distributor cap for moisture and corrosion on the terminals.
Use a small flash light to look through the throttle body opening to inspect for melted fuel line tubes.
If they are melted , install the updated design. It is cheaper than buying individual injectors, and runs better.
A scan tool that has the capability of reading live engine data is needed to see the cylinder misfire indicators. A P0300 is random misfire. that means more than one cylinder shows misfires. but there is usually one or two that show more than the others , and can be isolated with the proper tool.
You said you had 56 psi fuel pressure. Are you saying that is not the case?
Ignition concerns like moisture in the cap, carbon tracks on spark plugs, distributor timing( cam offset). these things when under a loaded condition will cause a cylinder misfire, low power and a backfire, when the spark plug starts to fire again, and there is a rich fuel mixture in the cylinder.
Connect your fuel pressure gauge up and drive it under the conditions of the low power and back fire. If it holds 55-60 psi, fuel pump is good. Fuel filters that are restricted can cause low power , but i have not had them cause a backfire.
#1 Look for a black carbon track, or a cracked spark plug insulator.
#2 inspect the spark plug boots for small pinholes. A small white burn witness mark on plug boot may be present if there is a hole in boot.
#3 Coil wire terminal connections.
#4 burn through marks in distributor cap.( look where two terminal come close together) this cap has terminals that cross the cap, and can jump through the cap material.
#5 P0174 rt bank lean fuel mixture. swap o2 sensors bank to bank and see if the lean condition follows the o2 sensor. ( it will set p0171) , if so it could be a skewed sensor and should be replaced. A sensor that creates a false lean condition , will cause the computer to richen up the injectors on that side. This can cause converter to overheat, and melt down, resricting exhaust flow. a pressure test can be performed on the cat to make sure it is not restricted.
These are the most common causes that I have run into . Just by changing plugs can cause the boot not to seal properly around the plug, This opens up the opportunity for moisture to get in and cause the spark to jump. Once it starts jumping to ground, it starts building a carbon track. The longer it goes on the more frequently it occurs.
If this is what you find, replace the spark plug and the plug wire, at the same time.
If you need more information, Please Ask.
Most people think they know what can cause a condition but never actually have repaired one. You were right for questioning that thought process. You sound like you have a very logical thought process, and I hope the information I provided you will be helpful.
Please if you need more help with this concern or another, Please Ask. I will be more than glad to help.