Obviously an older kettering system style car (C1,2,3).
Lots of issues. Are leads solid wire or resistive type. Solid = 0ohms, resistive = very high (meg ohms).
Try running up to temp at night, lift the hood in the dark and check for crossfire, arcing, breakdown.
Will appear as blueish crackles and flashes. Look at plugs if one is lightup or flashing, it maybe damaged internally.
I usually change Plugs, leads, points, cap ........AND the capacitor/condensor in the dissy.
If still faulty, only one thing left, the Coil, now you may also have a dropping resistor, which is by-passed when starting (gives full 12v hotter spark).
Comes into cct to drop volts to coil when running, as coil is designed to run extended periods at lower volts therefore cooler etc etc.
It is not unusual in electrical car ccts for heating to come into play after a time, any component can get hot enough to expand a crack or joint
and give an intermittent HR. This type of fault is the bane of the auto-electrician.
BOB