High speed engine miss and no problem at low speeds or mid range sounds like a lack of fuel pressure or fuel delivery. Ignition misfire occurs when the ignition is under its highest pressure ,sudden snap acceleration (the mixture goes lean) and starting from cold in the morning. The ignition may fail under prolonged engine load at wide open throttle. I wonder could the exhaust be restricted and not getting the exhaust out. High speed miss could also be caused by valve springs but this would be less common.