Been fighting this for a while. Engine only has a few hundred miles on it with fresh rebuild. Miss didn't seem to be there when I first got it running but engine didn't seem as strong as it should on the high end. Mummert heads and custom Isky cam. E carbs and original T bird distributor. Ignititon breaks down around 4,000 RPM. Had Pertronix I in it with the Pertronix coil. Switched out to a Pertronix III with the rev limiter  and the new matching coil. and same miss. The external resistror has been removed but it was in place with the Pertronix I. Checked the grounds and ok. Still missing. Pulled the plugs. Two fouled and a couple seemed to have a leak around the porcilin. Installed new different brand plugs and new Pertroncx plug wires. Same miss. Prety much shuts down, not just droping a cylinder. Had my retired mechanic friend take a ride today and he agrees it is ignition. I have and issues with the distributor in the past and have had bushings put in it twice since I bought the car in '65. Mehanic buddy suggested runing a jumper wire from the postive battery post to the positive coil termina to make sure its getting full voltage. Haven't tried that yet. Need to dig out my spare non tach drive distributor with points and see if the distributor is the issue. Any ideas on where else to look. Engine starts, idles, and runs strong up t around 4,000 RPM.