Recently I have been experiencing a problem on startup. If I shut down, and push the button for a startup; I get NO response. If I then disconnect the power cord and reconnect; I can then hold down my start button and the computer will eventually start. I must however constantly hold the button. When it does startup; I get a message to type in MAC-BOOT and hit the Return key. This causes a Restart. I have no idea what the problems may be. Please advise.