I've had this exact same problem on a '54 sedan with a 235. I replaced the condensor, points, coil, ignition wires, etc... and when the ehgine would heat up... the ignition system would seemingly just go *puh* and open up. readjusting the points would get me back on the road but when it would cool off, I'd have to readjust the points back to where they were. The timing seemed intermittent.
The problem was exagerated by the 6 volts... and what I found was that I was not securing the points down tight enough with that retainer screw... so when the engine would heat up, it would cause the points to 'slip' and open up. Since 6v has so much more current draw than 12v, there's more heat involved as well. And because I was always fussing around with the points on the side of the road, I in turn was not tightening them down enough because I was so frustrated.
The solution for me was a new set of points, a new screw, WITH a lock washer! Just make sure those points aren't going to travel anywhere.
