Sounds to me like maybe you have a loose connection. Jumper the plus side of the coil straight off the battery and see if there is any difference.
Also there is the possibility that the Pertronix is going bad.
1967 Shelby GT350, dark green, 347 stroker, 5-speed, inboards, #2206
1973 Mustang Convertible, white/white, black stripes, Q-code (351C-4V), ram air.
1968 1/2 428 CJ convertible, R-code, red/deluxe red, auto, finally finished.
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