@Reptoid Overlords is probably thinking there is a constriction somewhere, either in or out, that is limiting the engine speed. That is what I was angling for, too, but starting with fuel.
The fouled plugs indicate rich conditions unless the rings are so bad on each piston that you're getting oil into all of them. Too much fuel would overwhelm the spark, but I would expect the engine to die at that point.
What I was asking you to do in my previous post was to see if you could correlate fuel pressure to engine performance. Since you don't have the fuel pressure gauge installed, all of this can only be speculation. Data is more important. I was suggesting 500 RPM increments only because it would be easier to differentiate monitoring points. Ideally, you would make a video with a tachometer (from a timing light or multimeter) and fuel pressure gauge in the same shot, then we could correlate the engine sound, engine speed and fuel pressure. (A video done correctly can be very helpful for remote diagnostics. I knew I had an exhaust leak in my 240Z, and I sent a video slowly moving across the engine while the car idling to a friend. He said the leak was around #2, and he was right.)
So could there be a restriction in the intake? That is worth examining. Is the flap moving cleanly and easily, or is it getting hung? Is there something in the airbox or between the airbox and AFM (think critter houses) that could be blocking air flow?
Could the exhaust be blocked? Again, I would expect the engine to choke and die, but I could be wrong. I have seen several videos online of mechanics shaking a squirrel's stash out of a muffler.
Lastly, as I said before, make sure all of the wires on the coil, distributor cap, and plugs are fully seated on the components, especially the coil to cap wire. A weak spark could account for poor performance.
Since this thread is already 13 pages, would you please state again the age of the coil, cap, rotor, and wires? Are you on the stock ignition, or do you have a ZX distributor or other ignition?