Soak the CC on the mains again, trying at intervals to start from the keyboard.

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What exactly is this doing? Is it charging capacitors? If so, is it aging capacitors which have leaked and therefore cannot hold a reliable charge? It certainly acts like an old car battery that no longer maintains a proper charge. The car will start, but not reliably and only after sitting for a while, until eventually it won't even hold the minimum charge necessary to start it at all.