Thanks to both of you.
The ignition control module (aka power transistor) most likely isn't the culprit as it was just replaced with no change in symptoms. From what I can gather, the next things to test would be the ECU + wiring. Anyone else have some info?
Just FWIW, this guy had a similar problem (way back in 2003

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#/f/showthread.p...&highlight=icm