Issue with my 87 corolla

[BrockSRA]

I have an issue with my 1987 Toyota Corolla fx-16. I have a 4age bigport, and it is stock. When I drive for a little bit, suddenly the car will stop accelerating and wont let me go past around 3500-5500 rpm, and sounds like its running on 3 cylinders, or misfiring. When this happens, I have no power in that rpm range, so I'm stuck below around 3500 rpm while driving, no matter what gear I am in. However, if i shut the car off, whether I am moving or not, then turn the car back on, it runs fine for a while, accelerating normally as if nothing happened. Then, after around 20 minutes more of driving, it will happen again. This only happens when it is under load, as if I am in neutral, I can redline it with no problem. This has been happening for a few weeks, and I have had no luck with it. I have replaced my distributor cap and rotor, my spark/ignition coil, fuel injectors, and have switched out multiple spark plug wires, to no avail. If anyone has insight or has experienced this before, please help me.