When I first got my Corvair it had a Pertronix unit in it and it would intermittantly cut out and stumble while driving. I took it out and found that whoever installed it, didn't use the rubber grommet where the wires go through the housing and the insulation on the red wire had rubbed off, causing the wire to sometimes ground itself against the distributor. I taped it back up, installed a grommet and haven't had the problem since.
Ed