Good news is, the intemittent nature likely rules out the mechanical problems, e.g. wiped lobes. Could be sticking valves, but I would think it is electrical. Just out of curiosity, what condition are the plug wires in? Are they all separated and away from metal? It is possible the wires can be grounding out or crossfiring. Odd, but it happened on my 81 Mirada with cheap wires.
Also, the fire inside the cap could be firing the wrong plug at the wrong time. Hence, we want to see the location of the rotor relative to the cap and wires at TDC. As the distributor introduced mechanincal or vaccum advance, the position of the rotor inside of the cap at the firing event changes, and the spark has to jump the gap. If things are out of alignment (cap wires, reluctor in shaft, etc.) it can crossfire, and only occasionally at that!
Make sure you get a pic of the distributor, from the top down, with the cap AND rotor button off as well.
As the distributor advances, the rotor is at a different position and the spark has to "jump" across the gap, which it does even when everything is perfectly aligned.