I have a Crane ignition, not a Pertronix, so sorry in advance if this doesn't make sense. But I've noticed that it is extremely easy for the wiring in the Crane to restrict vacuum or centrifugal advance, if it's not installed with care. So, if there is such possibility with the Pertronix, it might be worth checking. I can't imagine any type of failure where it would work at idle but not at 2500 rpm.