Computer crashing all of the time, please help

First off I was not to sure where to post this, I hope i got it right. If not please correct me.

System specs:

Processor: Intel Core i5 4670K

Motherboard: Asus Maximus VI Hero

RAM: 8GB Corsair Vengeance Pro 2133MHz 

GPU: EVGA Nvidia Geforce GT 630

Storage: 250GB Samsung 840 Evo (Boot drive for Windows 8 and Linux Mint)

Storage: 1TB WD Blue HDD (Storage for windows 8)

Storage: 1TB WD Blue HDD ( Stroage for Linux Mint)

CPU Cooler: Noctua NH-U12S

PSU: Corsair AX760

My problem:

I built my PC about 3 months ago and it has been running smoothly until recently, when my system randomly crashed and then rebooted its self. I first put it down to a problem with Mint (the OS I was using at the time) and waited to see if it happed again; it did. I then switched over to Windows 8 to see if would happen in this OS and it did, however the crashes now became more frequent barely letting me into the BIOS for over 30seconds.

What I have done:

- I have unplugged all of my drives and it still crashes ruling out a problem with the OS

- Removed the graphics card

- Unplugged everything from the rear IO

- Removed all of the power cables from my modular PSU apart, apart from the 24pin power and 8 pin CPU power

- Flashed the BIOS to the original BIOS the motherboard was shipped with, using BIOS flash back. As well as flashing it to the latest release

I have taken all of these steps and i still cant get it to stay on for over 1 minute, I am now starting to think it is a problem with my PSU but i cant test today. So I thought it would ask for advice on here and see what you all have to say.

Thank you for taking your time to read this and please get back to me with any advice you have, it will be much appreciated.

could be that your cpu heatsink needs to be re mounted because it could be getting too hot and turning itself off

most likely heat like raging says 

You sometimes can read out the CPU temps in the bios. That would be an easy way to find out

I just managed to get it to boot for about 1 minute. It gave me a CPU fan error but the fan was spinning, i also managed to check the temps and it was running at 24-25 degrees Celsius (around 75 degrees Fahrenheit) before it crashed. I have had this system running for about three months now and i haven't had problems keeping the temps down before.

I have now got it running and its has been for 15-20 minutes, to get to this point I have disconnected the cases power on and reset switches. I will leave the pc running over night to test this but hopefully I am sorted now, thank you very much for your input it is much appreciated.

I would say it is quite possible that your PSU is the problem because we can rule out the drives when it crash on your BIOS. We can rule out the heat problem if you see 25oC as the temperature of the cores. The remaining possibilities are the motherboard and the PSU. I think you would prefer the second option to the first. Before changing the PSU or testing it, just be sure that there is no loose cables that could interfere.