open firmware problem installing to g4 imac

I have searched the web and tried suggested pram reset but it did not work. I am trying to load up tiger onto a 15" g4 800MHz iMac. I have tried a few different ways to install but it keeps giving me the open firmware screen after I hear it searching for the DVD to boot from. The DVD loads on the desktop OK and I can click the 'install' button where it then tries to restart, and I have also set the startup disk to start up from osx on the dvd but both won't work.

I have checked on the apple support site and it appears that this model does not need a firmware update. I did read somewhere that a security update could cause these sorts of issues but I don't know if that is the case here.

Is there anything else anyone may suggest I try?

I don't know if this is relevant but there is one 512mb ram module in one of the slots and one slot is empty