Kevin is probably right, but if doing his cleanup does not fix the problem connect a #12 wire directly from battery + to the + side of the coil and see if it "fixes" your problem. If so, problem is in the primary ignition wiring.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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