High Speed "surging" or "miss"

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While holding a steady 65-70+ mph going up steep hills (North Carolina)engine will surge or miss under load (can't tell which). Within last year replaced plugs, wires, ign cap and rotor. Within last five years have replaced in tank fuel pump, cage, external fuel filter, both pressure/return fuel lines and regulator in the intake plennum chamber BTW I just went on a 1k mile trip and got a solid 21+ mpg! No diagnostic codes are displayed.