I have an iMac 24" that has been having some problems. Every time I start up the computer, a grey screen appears and says "You need to restart your computer. Hold down the Power button until it turns off, then press the Power button again" (see picture). Seems like a kernel panic but I have usually been able to fix these kind of problems. I cannot, for the life of me, get this iMac to boot properly. I have tried the following methods with no success: -Reset PRAM -Reset SMC -Taken out RAM pieces one at a time to see if it was bad RAM. I tried each slot individually with no luck. -Tried to boot in safe mode. -Tried to boot to recovery drive. -Booted in single user mode and verbose mode successfully, typed in fsck -fy but no change. -Tried to boot using an install USB, both OS X El Capitan and Lion (10.7.5) no luck. -Tried to boot using Snow Leopard install DVD/CD. -Tried booting to external HD with OS X on it. I can hold down the option key on startup and it will show the Mac HD, Recovery, and anything else I plug in, but no matter which one I select, it will show a white screen with the Apple logo and will load about 1/4 of the way and then freezes. It will auto-restart after about 30 seconds of freezing or sometimes shut off. Sometimes it just gets stuck there forever. I have been able to put it into Target disk mode and go in from another computer. I moved the important files off of the HD, then used disk utility to check the volume. It says it is OK. I formatted and re-installed OS X but all of the previous symptoms still apply. I don't know where to go form here. I think I've tried everything that I know to do. I would really appreciate any help or suggestions. Thanks!