My iMac G5 (with Sight) won't restart

Question

My iMac G5 (with iSight) won't restart, every time I restart it the screen goes black and nothing happens, not even the startup chime sounds and the keyboard and mouse won't work, I can't even restart the PMU nor do anything, about a minute later the fans turn on making a very loud sound so I have to turn it down by holding the power button, after some time I manage to turn it on and everything works perfectly, but when I restart it again the samething happens, I've tried everything that appears in the support page, not even the hardware diagnostics detected a problem, even when I'm reinstalling Mac OS X when it comes to the part where it restarts itself the same thing happens.
Does anyone have a suggestion?
Thanks

Answer 1

Contributed by Fumiaki Kawashima;
There are many reasons to become unable shutting down or restarting from Apple Menu. Some were explained by other posters.
Most importantly, loginwindow (the parent process is launchd.) close out the user session, continue to set the device permissions and user preferences to their defaults in a restart action, or the process power off the system in a shutdown action. However, the issue proves that somewhere in those routines cannot be performed.
Users authentication system seems to be intact because you can restart or shutdown from the login window. However, the admin account authenticated by the Tiger security systems could confuse somehow and affect loginwindow process for setting up the user environment. This may result in interfering with the process for the shutdown and restart mentioned above.
Test if you can shutdown from the keyboard-- Holding down controloptioncommand keys, and then press the eject key.
Check if the loginwindow plists are corrupted. To do this test, remove the followings to temporary folders on the desktop, and shutdown from the keyboard shortcut combination. Restart the computer to test the recurrence.
/Library/Preferences/com.apple.loginwindow.plist
/Users/HOME/Library/Preferences/com.apple.loginwindow.plist
Try disabling Login Items for third-party applications via the Accounts system preference, if any.
Verify whether any third-party kernel extensions (including device drivers) interfere with the loginwindow hooks process-- Start up with a Safe-mode, then test the Restart from Apple Menu to see whether it works. Also, boot into a Single-user mode, and test if "reboot" command is available from the BSD layer. The I/O Kit API for software developers provides a shutdown hook so that ".kext" troubleshooting may need to be considered. Besides, some bad programmed extension may not be unloaded automatically at a shutdown or restart.
If the computer is configured with Irish language Input method and/or third-party Irish keyboard, try disabling them to test.
A third-party application or custom script may interfere with the loginwindow hooks for the Shut Down or Restart. For example, if you install an application that prevents casually powering down a system, the Shut Down and Restart buttons may not work. Also, if you install a P2P file sharing application, try uninstalling it.
As last resorts, troubleshoot a bad RAM and consider "Erase and Install" solution.
OPTION:
Post a part of system.log that indicates events from "localhost kernel[0]: standard timeslicing quantum is 10000 us" to the next same line.
This may help to narrow down because restarting or shutting down the system process can initiate the procedure programmatically by sending an appropriate Apple event to Mac OS X Tiger's system process (loginwindow).
Likewise, check console.log whether any failures is reported.
Corrections:
Most importantly, the loginwindow (the parent process is launchd.) close out the user session, continue to set the device permissions and user preferences to their defaults in a restart action, or the process power off the system in a shutdown action.
However, the admin account authenticated by the Tiger security systems could confuse somehow and it may impact on the loginwindow process for setting up the user environment. This may result in interfering with the process for the shutdown and restart as mentioned above.
Addendum:
If you post a copy of system.log, consider to edit the Ethernet interface ID to a fake value.
API = Application Programming Interface
Cheers!
DALE