I have an iMac (G4? with Intel Core Duo - about 2 years old). Lately once it's asleep or shut down, when I try to wake or restart it sounds like it's reving up but the screen stays black and the sleep light in front starts flashing on and off (very brightly) and won't start up. I usually have to keep trying to boot it up before finally it will. Any ideas on what might be going on?