I have an ibook G4 which is about five years old and randomly goes out with the screen going blank and all inputs ceasing to function. Closing the clamshell doesn't bring it to sleep mode. Hooking up an external display at that point yields no input from the computer. The hard drive and fans still run, yet when I shut it down (holding down the power button) it seems to work just fine. Then about an hour or less, the same thing happens. This has happened three times so far, but it could get worse.