Question POST screen loads, but then computer resets and won't let me into BIOS config

[DarrenGrey]

I'm really struggling to figure out what's wrong here - any advice appreciated. My computer (self-build about 4 years old) started not booting properly - it would start doing the following:

1. Turn on
2. Turn off after a few seconds
3. Turn itself on again with all fans whirring and reach the POST screen
4. Switch to a black screen with a mouse cursor and flashing key cursor
5. Reset back to 1

Oddly on the POST screen F12 will dismiss the screen but not take me to the BIOS config.

I have tried:

• Resetting CMOS
• Replacing the motherboard battery
• Removing the memory sticks, causing the motherboard to continuously beep when turned on (a good sign for motherboard health, at least)
• Removing all SATA connections and PCI-E cards and using only motherboard HDMI out

None of this does anything, apart from CMOS reset which seems to have stopped the POST screen from displaying (though the rest of the process continues to happen as above).

Any ideas? I'm going slightly mad here. Motherboard is Gigabyte GA-Z97P-D3. I've searched all over the internets and can't find a working answer.

Edit: Adding in further system specs:

PSU: Seasonic S12 II Bronze 620W
CPU: Intel i5 4590. Has a Zalman cooler.
RAM: 2 x 8GB Corsair DDR3 1333Mhz
Other components aren't connected anymore, but include NVIDIA GeForce GTX 760 GPU, 2 SSDs and a regular HD.

Edit2: Solved by replacing motherboard!

 

[rgd1101]

full system spec? include make and model of the psu

 

[DarrenGrey]

full system spec? include make and model of the psu

PSU: Seasonic S12 II Bronze 620W
CPU: Intel i5 4590. Has a Zalman cooler.
RAM: 2 x 8GB Corsair DDR3 1333Mhz
Other components aren't connected anymore, but include NVIDIA GeForce GTX 760 GPU, 2 SSDs and a regular HD.

Will edit into original post.

 

[rgd1101]

test with another psu?

 

[DarrenGrey]

Tried another PSU - same problem. Eventually managed to get it fixed by replacing the motherboard.

Kinda bizarre, I've never had a motherboard fail before.