Hi drwhose,
I have the same motherboard with you, could you mind to share your bios setting with me?
When I use boot the computer with the Zephyroth Leopard 10.5.1 or 10.5.2 it hang.

-v -x it shows:
EFI enhanced bootloader build: ToH
Using SMBIOS table found at 0x000fbdb0
Using ACPI RSDP revision 0 found at 0x000fa7c0
Hey we are going backwards! -> , restarting timing
Hey we are going backwards! -> , restarting timing
Calibation failed with 2 attempts

THANKS~!