Hey, but if your machine is a PowerPC, grub do not apply. Openfirmware is the boot loader instead. My guess is you have a corrupt kernel. Any way, after you select linux (presing "l"), hit TAB key to see what kernel are available, and then type the one without the "*"
Post the results here
Pietro Pesci Feltri
PowerBook 15" G4 and
MacBook Pro 17" Intel Core 2 Duo
Intel I5 Desktop