This could be caused by a USB device attached to the computer, an extension conflict, or an item in either the Startup Items or Shutdown Items folder.
Follow these steps to verify:
Disconnect all USB devices (including keyboard and mouse and any USB hubs) and power down the computer as follows:
- iMac – use the front power on/off button to power down.
- Power Mac Cube – hold down the power button for about 5 seconds to power down.
- Power Mac G4 – remove the keyboard and connect a known good Apple Pro Mouse and shutdown the computer via the special menu.
If the computer shuts down without restarting, it is likely the fault of one of the USB devices. Connect each USB device one by one and repeat the shutdown process until you identify which USB device is causing the fault. After identifying the suspect USB device, contact the vendor of the USB device for further instructions on resolving the issue.
If the computer continues to restart even after disconnecting all USB devices, then it could be caused by an extension/control panel conflict, or an alias in the Startup Items or Shutdown Items folder. Remove any items in the Startup Items or Shutdown Items folder. Restart the computer and shutdown to see if this was the issue. if the computer continues to restart then make sure the issue is not caused by and extensions conflict.
Ah yes, extension conflicts.