Hello, I realize this thread is old, however I have the same issue, EXCEPT NO error in the log regarding the fan. In fact, no applicable errors. THe system abruptly shuts off like the cord was yanked. System diagnostics runs in loops with NO ERRORS. I am at wits end. Powermac G4 A1047 4GB ram. Thank you. Update: Well, nobody ever answered, but I think I found the issue. It has something to do with certain USB devices that OSX has problems recognizing. (even though they function, OSX cries about it in the logs) As soon as I removed the offending USB device(s) the issue went away. Machine has been running for a few weeks without issue.