Well the mystery deepens have decided to try a 3rd higher rated PSU and would you believe it all running well again ???? Had better soak test over a day or 2 as it could still be the Motherboard playing tricks on me !!!!
I sometimes work on desktops given that they must be easier than laptops, don't be fooled! Something I didn't see posted yet was you can rule out a cpu problem if you can post. A faulty cpu will never post or at least from my experience. Heat issues can cause those intermittent power offs but they would be consistent, meaning it would happen all the time till the cause was fixed. I searched an image of the motherboard, and it looks to have solid caps so those wouldn't bulge I think. I once had a bad case which a short on the pc speaker that would blow my psu but it was a gradual process for some reason. I've had 2 asus m2a-vm flake out on me, so the brand itself is now tarnished for me.
So far has been switching on & off fine. Left switched on all day yesterday all good ? Cannot understand why this 3rd psu has made the difference I cannot have fitted that 2nd psu to have exactly the same fault as the original this is what threw me and I suspected the Motheboard ? I still do ! But so far so good ?