Let me start by saying I have swapped the ram in the computer and the problem persists. I've also tried resetting the PRAM. I have unplugged all devices except the keyboard and mouse. I have reformatted and installed OS X Leopard, Tiger and Snow Leopard. I have even encountered this issue while the computer was running from the install disk. I'm pretty sure software, ram and third party devices can be eliminated from the possible causes. The computer continues to run after the screen goes black. I am connected to it right now using screen sharing, so I know the computer is still working. Putting the computer to sleep and then waking it up temporarily brings the screen back until its next random blackout. I have opened the computer to replace a dead HD, so I suppose it is possible that I knocked something in the process, but I did that back in december and the problem is a relatively recent development. I would really prefer to fix this myself, if I could only figure out what the problem is. Any ideas?