[Oldsguy]

conduct a physical inspection of the parts changed during the tune-up. Look for a loose A+ wire on the coil, make sure it is tightly attached to the stud on the coil. Make sure all your plug wires and coil wires are inserted fully into their respective tower locations. Make sure the distributor hold down is tight. Make sure the points are secured to the backing plate well. Inspect the A+ wire from it's point of origin to the coil (actually back track as far as you can go from the coil backwards) look for a place where it might be touching something hot or have cracks or nicks on it. The only reason I am mentioning these is that I really don't have a singular item to identify and what I recommended you do is what I would do first. It might take up a half hour of your time and you might find the culprit.