Back in Dec. '99 I bought an iMac DV 400. Recently, the monitor started flickering, and wouldn't start up at all sometimes. I could tell that the computer was on because I would push the power button, and it would ask me if I was sure I wanted to shut down. This morning though, when I hit the power button, the iMac would make the tone, the hard drive would spin up, and then it would just die. What should I do?