I have a Power PC G4, OS 10.4.11. Today, it started going into sleep mode while I was working on it. Hitting the space bar would bring it up again, but it would just go back down again in 15 seconds or so. I eventually navigated to shut down and kept waking it up long enough for it to shut down. If I leave it off for a while, I can turn it on and work on it for about 10 minutes or so before it starts repeating the process. What and why is it happening?