High speed miss under load is usually ignition related.
Since you have changed the plugs, & distributor cap, either the spark plug wires or TFI module are the likely culprit.
Wait until dark and do the "Do I see sparks test". Open the hood and start the car. Watch the plug wires for spark jumping or glowing around the boots along the wires. If you have an automatic, have someone run the engine up against the brakes and do the test. Just be careful that they don't give it too much throttle or let up on the brake. If you have access to an Volt/Ohm meter, check the resistance of the wires. They should be about 2000 ohms or less per foot of lenght.
The TFI module is mounted on the distributor on 86-93 year cars. When you know for sure that the cap, plug wires and plugs are good, it is the most likely source of a high speed miss.