Nice car! Once it starts with the jumper, does it keep running with the jumper taken off? Sounds like you have a bad connection somewhere in your ignition hot wire. Should be black/pink on a 69, When the circuit gets damp it causes enough resistance that the current flow stops. May want to take the wire off the coil and check the terminal connection, could be that you have to crimp on a new one. Also check the wire all the way back through the circuit for splices that may be loose or corroded. Or it could be moisture in the ignition switch.
