Shipping to here would kill it for me, but whoever does pick up this potential gem could try booting a Linux Live CD or installing Linux on it and running from that for a while.

It might give different information if/when it locks up, for one more clue in the puzzle. Once that's sorted, go back to OSX for the long term, if desired.

Fedora and Debian both have PPC versions of their Linux distros, and there are others out there too, including Ubuntu 6.06.1 LTS.
Cheers