rotor phasing is when on some dist's vac & centrifugal adv & stackup of factory tolerances has the rotor tip so far away from the terminal on the underside of the cap that it is firing to that the spark cant jump that far & it starts to miss. you could drill a 3/8" or so hole in the dist cap between the center terminal & #1 terminal rev the eng up to the offending rpm & shine your timing light straight down in there & see how far off the rotor is from being close to the underside of the #1 terminal. easier would be to advance your initial 5 degrees & rev it up to the problem rpm then retard it 5 degrees(10 degree change) & rev it up there again. one of these 2 actions will move the rotor closer to the terminal & if phasing is causing your problem it will get better.If so holler