I recently had a similar problem with my iBook on OS 10.4
Many things I tried would work for a short time, but the ultimate solution ended up being to boot the iBook into FireWire Disk Mode (Holding gown ‘T’ at the startup chime, if I remember correctly), attaching it to another mac with a firewire cable, disabling permissions on the mounted HD (Get Info > Ignore Permissions on this Volume) and copying my files out before formatting and reinstalling.
Hopefully you won’t have to resort to such a drastic move, but it ended up being the only thing that fixed mine.