From my experience working on these, it’s commonly the DVD/CD drive that goes bad, I’ve had to replace several of them.
# You could try is booting from a Firewire 400 external hard drive with OSX installed on it by holding down the alt/option key when starting up.
# Or you could optionally connect it to another Mac that also has a Firewire 400 port and put that Mac in “Target Disk Mode” by holding down the T key at boot.
I know this post is old, but this is for anyone else that might have this issue in the future as it might be of some use to them.