Rotor phasing: If the rotor tip gets far enough away from it's terminal on the underside of the dist cap the spark wont make it everytime & it'll effect a miss. a combination of initial timing/mech adv/vac adv and or a slight machining error in any one of these, cap/rotor/adv plate/reluctor can cause a stackup of tolerances. I drill a 3/8" hole in my dist cap in the flat top part in between the center terminal & the #1 terminal so that I can see the rotor & I freeze it w my timing light at the offending rpm & see how close the rotor metal blade is pointing to the center of the #1 plug terminal. Or adv and retard your initial 5 to 10 degrees from where it is now & see if that affects the miss as one of those 2 actions(adv or retard) will put the rotor tip closer to the bottom of the dist cap terminal.