I've seen this before, although it was very difficult to diagnose and track down. It was also on a 1960s Ford Galaxie so it had a few cylinders to spare and the owner was content with driving on 7 for the first mile!
So basically it turned out to be related to thermal expansion, either the cap, rotor or both were cracked or damaged slightly but invisibly, so proper connections were not being made until the temperature increase closed the gaps.
It might be worth replacing the rotor and cap for the purpose of elimination, as they are very cheap. Or borrow one from a known working car if you can.
I'd also be checking the resistances on the HT leads to see if one is being temperamental.
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