For anyone else heading off on a similar voyage: it's almost always the power supply. And then the faulty power supply kills the ICs. Electrolytic capacitors

For anyone else heading off on a similar voyage: it's almost always the power supply. And then the faulty power supply kills the ICs. Electrolytic capacitors are unpleasant things. They require a liquid that slowly evaporates. They're temperature sensitive. Degraded ones behave erratically when they warm up, temporarily behaving and then not. And they can, worst of all, fail short. Quite possibly why your Apple's resurrection was temporary. I'd be more surprised if there wasn't a bang and magic smoke dissipating after turning on a PSU that had likely been off for 30+ years. Always replace or reform them in equipment of that vintage.