Power Mac G5 2.7ghz error message "You need to restart your computer"

We have a late 2005 Power Mac G5 2.7 ghz running on 10.5.8 OSx with 4 GB of memory, that will not run for more than 10 mins without crashing. We just get a dark grey screen that comes down and then shows a message in four different languages that says "You need to restart your computer. Hold down the Power button for several seconds or press the Restart button."

Would be grateful for any suggestions on what could be wrong and how to fix it as its unuseable in the current state?