I've been seeing similar issues, mostly kernel lockups,
with 1.6ZG on a G4 1GHz. The box was being used
as a build machine for our embedded port (MPC8245
based). It would freeze every few hours whilst under
heavy load.

I noticed similar behaviour on the 8245. I caught a
panic in uvm_swap_io (the board was NFS booting,
and mounting swap over NFS). Disabling swap space
stabilised things ... doing the same on the Mac also
seemed to help.

Sadly, the harddrive in the Mac died over Christmas.
Once I get it back up I'll try and probe further. I'll
also investigate some more on the 8245 boards.

Cheers,
Simon