Slide a pierce of stiff, clean paper like a business card between the points, and see if any carbon or other deposits get transferred to the card stock. Also double check the point gap- - - -if the rubbing block wears it can close up the gap to the point the engine will barely run due to late timing. If the points check out OK, do a close examination of the wire running from the coil to the side terminal of the distributor. Since the housing is in constant motion from the vacuum advance, that wire can develop broken strands inside the insulation that effectively turn the ignition on and off as the distributor housing moves. I like to replace that wire with a piece of high-flex wire scavenged from a discarded voltmeter test lead.
Jerry
