Fouled ignition points. Those cars used mechanically triggered transistor ignition and had almost no current flowing through the points. They insulate with the slightest smudge.
I have had brand new points out of the box leave me on the side of the road more than once. Drag a dollar bill through them and then grind them together (be sure they are closed first of course).
Bosch Master, ASE Master, L1
33 years MB technician