My new (to me) G4 runs fine (OS 10.5.5) except that after about 45 minutes, it just quits and returns to the login screen. I first noticed this as I was attempting to update software. Download proceeded, and after about 45 . . . back to login. Sitting, running unattended, with all power save options set to "never" it does the same thing. Any ideas. Thanks in advance.