IIRC, the spark box was a finned aluminum deal; they could be trouble-prone.
I recall replacing one decades ago on a buddy's '79 Ford Ranchero.
Same symptoms, and I don't recall anything visual like leaked potting material.
"EEC-IV" (IV as in 4) comes to mind...

A LOT of water's passed under the bridge since then, so I could be off on the Gen 4 nomenclature.