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.