I would check the tooth/magnet gap first. Speaking of the gap not RP you set it at .008" at rest (eng off/no vac) & the can increases it but not to the point where it will misfire but if the gap is too much greater than .008 to start with at rest then when the can kicks in it will increase it past it's "limit" where it will then miss with the can activated. The can changes reluctor gap and RP. A stackup of tolerances in the wrong direction from poor machining can shift it (RP) past it's (functioning) limit.