I purchased an used, Apple refurbed Early-2008 Mac Pro (8-Core 2.8ghz x 2, ATi 2600XT, 2GB ram basically completely stock) and Snow Leopard. It is currently out of warranty. At the time of purchase the previous owner told me that the computer wouldn't start and that a power supply replacement would remedy that. After getting the computer I took it to the Apple Store and viola, it turned on fine. It seemed to work fine for a few days but then had trouble starting. Here are the symptoms: -The computer powers on, the video card fan turn on and it would reach the Apple logo with the spinning thing below the logo and then shut off. OR -The computer powers on, the video card fan turns on and then the machine shuts off before the monitor comes on. -If I leave the computer completely unplugged for a couple of days, it does boot fine and reach the desktop. I'm able to maybe work for a few minutes (sometimes it does go for a few hours) before the machine shuts off without warning. Oddly, this seems to only happen if the computer is off for a few days, leaving it off for a few hours doesn't seem to do the trick. I have already tried resetting the SMC and removing the memory and video card and putting it back in both the original and different slots. While the computer was able to stay on, I did run disk utility and the hard drive checks out ok. I tried reading different threads on this issue and it seems like it is one or a combination of the following scenarios: 1. Bad logic board 2. Bad power supply 3. Bad video card 4. Bad hard drive Based on my symptoms, can anyone offer me some knowledge and advice on what the issue may exactly be? If it is a bad power supply, would it even come on occasionally, etc etc. Thanks in advance for any help.