(1) rotor phasing (2) too much timing. with the can plugged in see what your timing is at at 3K (where it is acting up). Unplug the can & set it at 3K & crank the dist till the timing is back to where it was with the can plugged in so the RPM is the same & the timing is the same just that it ain't coming from the can. If it's OK now then it's rotor phasing. Only the can alters rotor phasing & since you need the can for street use there's several ways to reclock the rotor & the NAPA rotor may reduce the gap distance back to where it wont misfire. To rephase. easiest is to cut another rectangular notch in the top of the dist shaft where the rotor notch locates (carefull measureing required/measure twice (at least)/cut once. It's strong enough there to handle one more notch. NAPA offers a rotor (MO3000) for $8 & change out the door with a .060" longer metal blade that reduces the radial gap from rotor tip to cap terminal(s) & helps with circumferential phasing issues as it is the total gap that once it gets wider than the spark can jump then it misfires