iMac G5 Freezes After Startup Even After Hard Drive Replacement

My Imac G5 was freezing up during or after start-up. I sometimes reached the desktop and it would freeze. I got onto the internet a couple of times before it froze, as well. I also got the lines going across the screen (in random/intermitent patterns). I found a few sites that stated the problem was likely the hard drive going bad, so I replaced the hard drive and still get the same problems. I tried various scans and fixes like the ones in Disk Utility, but still the same problem with the freezing and lines across the screen. The hard drive is now a new one, so is this still a hardware issue? I even tried a fresh OS install on the old hard drive (no changes) and the buyer of the new hard drive did a clean install of Snow Leopard.