the funny thing about ps3 is one symptom can be caused my many diff things. i had a slim that had this same problem and i reflowed the rsx and cell with no success, then i reflowed the southbridge BGA chip and it made it work but it eventually did it again. im not saying this is your problem as like you said this can be a bad psu, bad resitors. bad caps, bad fuses, bad bga chips... you name it.
another thing you can try is to ground out tristate and see if it stays on. that could mean it is a software problem and not hardware
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