[snvzz]

It was the power supply, no doubt.

Back at the time, we (little me and my family) already knew it was bad. But we weren't aware of its failure modes. All we observed when measured was low voltage, and that it "sometimes worked".

That's why so many C64s were damaged, and still are, when people connect their old supply without the knowledge (or against warnings) of such a nasty design flaw.