general rule of thumb is that any time you want to probe or test your machine (from the ok prompt) you need to disable the auto-boot nature of the machine and start from just after a clean POST.
You can switch this on or off at either the ok prompt or from a running system.
ok> setenv auto-boot? true
ok> setenv auto-boot? false<just toggling back and forth>
root# eeprom "setenv auto-boot?=false"
root# eeprom "setenv auto-boot?=true"<same thing from the running OS>
hth
Jeff