1969 dart loses spark once it warms up

This is tough. Could be a number of things

1...Go to MSD site and READ the troubleshooting prodecture for the MSD

2...De-wire the MSD and "hot wire" the box, making CERTAIN that the big red and big black are well hooked to a good battery source and ground, and then "hot wire" the "small red."

3...Just try a spare coil. Coils can be intermittent, and back in the day, this would be a sign of a bad coil

4...Make DARN sure there is NOTHING connected to the coil, NOTH---ENG except the 2 wires from the MSD box

5...Any evidence of overheat/ moisture damage to the box/ coil?

Remember that MSD works nothing at all like a conventional "switched" or Kettering points / electronic ignition. These girls work more like a photoflash. They charge up a large capacitor to high voltage, and then the trigger dumps the charge across the coil.