speedometer question& ignition question

I had this exact same ignition issue on my 68 Valiant when I first bought it.
It would start fine and run as long as I kept presure on the key (half way between running and starting)
I replaced the ignition,then the resistor, then the coil but nothing changed.
What finally fixed it was putting in a new distributor.
Not sure to this day why but the distributor I took out did have an aftermarket pertronix in it and the new one came with good old fasioned points.