I guess that happens after loading boot.img, right?
In most oft that cases the problem seems to be poseidon.prefs (USB-Stack). Try to delete ENVARC:Sys/poseidon.prefs and reboot without any further action.

Some hint to G5:
It is very helpfull to have a "emercency" installation on a CF-Card, fittet als slave to the internal IDE-Bus (CF2IDE-Adapter required) - the DVDRW is configured as master and you should'nt touch that config (will give troubles with Open Firmware device aliases)
If that CF has a small HFS-partition including boot.img and a big bootable SFS-partition with MOS installed (called "CF1" or anything else), then you may boot as follows from Open Firmware-prompt:

> boot cd1:,\boot.img bootdevice=CF1

Don't give CF1 a higher boot-priority than your normal boot-partition, because MOS tries to boot from the partition with highest priority (as old AOS does).

Now you are in a separate MOS-Installation and can "repair" your main-installation.....or use it as origin-testinstalltion for games or other software.