76 280 will just shut down. Happens randomly at any speed. When restarting it will start/stop 5 to 10 times then runs fine for anywhere from an hour to a week.

Tach needle drop first like rock or follow engine speed as car dies? Ignition versus fuel.

Check the wiring near the ballast resistor / coil

Also check the wire going from the coil to the dizzy, thats what happened to me once the wire was stuck up inside the rubber

Check the 2 fuseable links on the battery, I just found that mine were killing power to the ECU.