Hello! I have a similar issue: sometimes the clamshell doesn’t turn on for hours, then it comes back to life again.
I have a newertech battery which I thought was dead, but it came back to life as well after a 4 years of non-usage - now I get 5 hours off the battery! But it would never start just from the battery (if it starts), only with the power adaptor plugged in, And, another weird thing: the only way it would charge the battery is when in sleep mode: not when it’s turned off completely, and neither when the lid is open. But well, it still DOES work mostly, I have opened this one several times (466 firwire, 128gb hd, 576 ram, double boots Tiger and LinuxMintPPC).. I was thinking about replacing the charger board but I also know that there’s so much that can break during the process. I also read about this capacitor battery, could thatr be the issue, and how difficult is it to replace it?