Apple today released Power Mac G5 (June 2004) Firmware Update 5.1.8f7 which improves system stability for your Power Mac G5 (June 2004).
To upgrade the firmware on your Power Mac G5, follow the instructions below. You can print these instructions so they are available while you upgrade your firmware.
The Power Mac G5 Firmware Updater is installed in Applications/Utilities.
1. Save changes to open files and then quit all applications, if necessary.
2. Click Shut Down in the Firmware Update window to turn off your computer. Wait for your computer to shut down.
3. Press and hold in the Power button on the Power Mac G5 until you hear a long tone or see the Power button light flashing.
The firmware updater starts automatically. A status bar shows the progress of the update. You may see more than one status bar. After the update is complete, your system restarts automatically. When a message says your computer’s firmware has been successfully updated to version 5.1.8f7, the update process is finished.
More info and download link (1.2MB) here.