Hello there, I have been using Mac systems for a while now but have never faced problems like this before. I have a Mac Pro which has Mac OS X 10.5.3 operating system on it. Recently the Mac Pro has started giving unusual problem. Whenever the computer is started, it works fine for sometime and then it suddenly shuts down. After shutting down I cannot start it for sometime and then it starts normally when the power button is pressed. It is very annoying that the Mac Pro shuts down suddenly. Can this be due to operating system as I tried to upgrade the operating system but quit the process midway. Or is it an hard ware related problem?