Hi roisoft,

I also use this motherboard (GA-M55Plus-S3G),
the bios version is F13 (the most latest one...)
I disable Serial, Parallel, Firewire, SATA, Second IDE, Use S1/S3 power
set the HD=master DVD=slave on primary ide channel
When I 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

How can I solve this problem?
THANKS!