ACPI problem on Fedora Core 8



If I set on the bios ACPI=yes , the O.S. with kernel 2.6.26.3-14.fc8-
x86_64 doesn't boot :(

The error is :

ACPI : Expecting a [Reference] package element, found type C
usb 1-2 device not accepting address 2, error -110
usb 2-2 device not accepting address 2, error -110
[...]

If I add acpi=off in grub.conf Fedora will boot, but I can not halt
the system anyway !
Infact If I execute poweroff on the prompt, I see "the system is
halted" on the screen, but PC and monitor are aòways on.

Could you help me please ?

Thanks

Claudio.
.