Will not boot from hard drive or Install Disc

I have an editing system built around this old Power Mac G5 late 2006 2.33 core 2 Duo (pre Intel). Several other machines in the building access our Apple Raid array through this machine that has fiber connection to RAID. It's essentially the Mother Ship for the entire house.

I ran Cocktail utilities on it before leaving a couple of days ago with command to shut down upon completion. I do this occasionally before weekends or holidays. When I tried to restart the machine 2 days later, I got as far as gray screen with apple before all fans started whirring.

I've tried safe boot, zapping PRAM, booting from OS disc to run hardware check, removing fibre IO and AJA Kona (edit IO) boards - nothing works. Sometimes I have gotten as far as the language selection menu of Apple hardware test before the mouse freezes and fans begin taking off. Just checked the voltage on the backup battery. All OK.

LEDs on logic board (that I can see) show #3 red followed by #4 amber and sometimes #5 green. Suggestions greatly appreciated. Thanks.