Maybe it is that it's a slim I've just tried a System Restore (only a quick format though) and reinstalled the System Software to no effect. I've read in a few places about some kind of compatibility downloads for some games not sure where these would be found though. There's no settings people have had to tweak to get PS1 games working?
I've finally managed to dig out another PS1 game (I don't have many) Bloody Roar, and that's loaded without problems. So at least I know it's just FFIX causing the issue