I was rereading the responses. Said to myself, this may be a grounding problem. Check the grounding on the dist, coil, the ground from motor to frame,battery ground , etc. Make sure you check the wires, connectors, and connector surfaces.May sound stupid, Iworked as EE for 36 years,amazing the problems of bad grounding.

1954 Chevy Half-Ton
In the Stovebolt Gallery
More pix on Shutterfly
More pix on Photobucket

1958 Chevy 1/2-Ton Stepside
In the Stovebolt Gallery
More pix on Photobucket

<> + <> + <> + <> + <>
"Time's fun when you're having flies." ~~ OLD FROG