Breakthrough after 45 days!
Today system powered down and would not start by pressing front power button.
I used the power switch on the PSU to switch off, but when switched on the computer would automatically start for 2 seconds then switch off.
The fault was permanent and gave me enough time to substitute my memory, CPU, and graphics card onto my spare motherboard, but still fault remained.

I thought I had the same fault on two motherboards, there was only 1 thing I never changed and that was the tower case. I'm using Nexus Clodius which has since been discontinued:
http://www.quietpc.com/nx-clodius

The case has builtin fan with variable speed control. This has a standard 4 way power connector (only applies power to case light and case fans). Removing the connector made no difference, PC would still start as soon as PSU switch was on.

I then removed all the header wires from motherboard connector for HD LED, reset switch and Power switch and this time when power switch on PSU was pressed, the PC would not start.

This strange intermittent fault has been down to a failing front power button. I dismanted the speed control and had a look inside. The front power button is only a non-locking push-to-make switch. There was some dust and contamination which I have blown away with an air duster. I have operated it on and off about 20 times (while PC is off) and believe this action may have
helped clean contacts and possible remove any dust or dirt on switch contact spring (although
some cheaper switches may have no spring only mechanical contacts).

I now am pretty confident this is a mechanical switch failure than hardware and can explain most of symptons:

Mysterious shutdown: The default BIOS Power management setting for the Power switch is instant off. If I press the front power switch then KDE will just shut down. (It can be changed in BIOS to 4 sec delay) I'm sure that the front switch contacts must have been close enough to touch, just through vibration when the computer appeared to shut itself down.

Power On: Occasionally PC would start on its own, again I put this down to a fragile or poor mechanical front case switch.

Boot Failure Message in BIOS: This is the one message that makes the fault look like hardware, PSU, CPU, motherboard. The message was "Your computer has experienced Boot failures due to overclocking or voltage changes". I think the prominent words here are "boot failures" and again think this is nothing more than contact bounce on the front case switch. If pressed on, then off, on then off before BIOS has loaded I am sure the same message will appear.


I also have a simple test for suspect front case switches, a light flick of the finger somewhere near the switch should not affect the computer. However if the switch is faulty and BIOS power management is set to instant off, then your computer may shut down and would indicate a possible
faulty switch.

I'm not out of the woods yet, as I need to test my system for a number of days.
However the fact that the PC would come on with just the PSU switch without pressing the front power switch, proved that the fault was front power switch contacts stuck. If I have no more problems for a few days I will mark thread as solved.

I hope this helps someone, thanks to everyone at LQ for support.

Update:
I am marking as [solved] now because I am 100% confident of the cause and can account for all failures.
I have now been able to recreate a boot loop, one of my symptons.
Simply press and hold (dont release power switch). Computer will start up then shut down (after 1 or 4 seconds depending on BIOS setting, Power management). A boot loop can be caused (in my case) by sticking contacts on front switch or contact bounce. Even possible that in some cases the wires may be earthing or
shorting.
After creating a boot Loop and if you use Award Bios v6 you will see a message
"Your system has experienced boot failures. This may be due to overclocking or voltage changes. Your current hardware may not match these settings"

Voltage changes you would suspect a PSU, or motherboard, even CPU. Of course the message also states "boot failures" The boot failure was caused because the BIOS never fully executed all its routines before system was shutdown.

The front switch may not be the cause of all boot loops, random shutdowns, and random switching on but its quite a common report now on google, and could happen to laptops and notebooks as well. Hope this helps someone.

1 members found this post helpful.

That was definitely an expensive and difficult to diagnose hardware issue. It seems the reboot loop is the most characteristic.

What made this more difficult was the time between failures, sometimes 9 days, sometimes 7 days, then as little as 1/2 hour.
The contact bounce was responsible for random turning on and random turning off. At first I could not work out the reason for the reboot loop, but after later checking my BIOS, power management the Front Power switch delay was set to instant off 1 second. When the reboot loop appeared, it was just that the case switch contacts were permanently on, so it would turn on, turn off, turn on again.

Although a switch is a weak component, I never use the front switch much, as I switch on and off mostly from the keyboard,
which has power, sleep amd wake buttons. I do remember though, when my case arrived that one of the front fans was shaken loose, so although no physical exterior damage, its almost certain to be why the front switch has failed.

It may not be the answer to everyones problem, but the switch was someg I never even considered could be the problem. Possibly a shop reapir would have been many times more expensive than me buying another motherboard from ebay as well.