I am thinking it is your ignition. One way to test, is to bench test it. I use a drill press to spin the distributor and a piece of metal drilled and threaded for the plugs. Use the coil and ignition box, ballast and battery from your car. You need to use heavy wire to connect the grounds at distributor, box, and plug metal and battery (-). The distributor turning at 3000 RPM simulates the engine at 6000. Viewing the spark plugs may show the problem. I view the primary voltage with a scope to see the dwell and peak voltage. Depending on wiring, you may have noise issues related to spark and distributor pick-up, or low voltage condition creating the miss. A spark plug wire close or contacting the pick-up wires could be a problem. Start at a lower RPM first in insure all is fine.