I am about at my wits end and don't know what to do. I have a late 2007 iMac 7,1 that had symptoms of freezing and locking up with Mavericks on it. I was told the Hard Drive might be going so I replaced it (it was under warranty) but the same symptoms occurred... Locking up where the mouse pointer moves but you can't click on anything. Then when you do a hold the power button down reboot, it never fully boots up and the screen stays gray. I then booted up into Safe mode and downloaded El Capitan and installed that in hopes of it fixing this problem. To no avail, the computer locked up a day after I loaded Cap on it. I can however hold the Shift key down and boot into Safe mode and everything works but slower. Does anybody have any idea what is wrong with my computer???? We really can't afford a new one right now. Thanks in advance.