I just upgraded a Core Duo iMac to Snow Leopard. It worked great until the moment when I installed Snow Leopard. Now I have random horizontal lines, crashes, and hangs. At the Apple store they hooked it up and did diagnostics. All the hardware is operating properly according to Apple. I have reset PRAM, reinstalled the OS 3 times, erased the hard drive and did a clean install, repaired disk permissions, repaired hd. It will work pretty well in safe mode, so that tells me it is a hardware issue. Any suggestions? I'm losing my mind. And no, it is not covered under Apple Care. And no, I don't have a copy of Leopard.