do you have the cds that came with the mac? if you do, try booting from the cd. do that buy turning it on, put the cd in and turn it off again. then turn it on, and as soon as you hear the bong, hold doen the "c" key until you see the welcome to macintosh screen. if you can't get it to boot from the cd, or it does the same thing with the cd as it did with the hard drive, then i'd say ram or motherboard problems. if it seems to run ok, then try opening the hard drive. open & close several folders, moving around in the file system. if it seems to be working right, it may be a bad system. if you are having problems, it may be the hard drive. if you have over 400mb free space on the hard drive, then run the os 9 installer and have it do an archive and install, or it may of called it something else, but it will have an option to rename the old system folder, and install a new one. try that, and reboot. if all seems well, then it was a bad system. if not, then it may need a new hard drive.
Digital Audio G4/1.467ghz, 1.5gig ram, 16x Superdrive, 256mb DDR3 AGP 6800GS, zip, 2x500gig raid0 for 1tb on sonnet tempo trio, 10.5.4