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I'm hoping that somewhere out there is a wizard who can help me save the day.

Last night I decided to o/c my i5 2500 and things have not ended well. But firstly, some specs:
CPU: Intel i5-2500k
Mobo: Asrock P67 Pro 3 with 3.3 firmware.
Video: R9 270X 4GB
PSU: Seasonic 520W PSU
Case: CoolerMaster CM 690II
HDD: Primary - Intel 120GB SSD. Others - 250GB SSD, 1TB SATA, 500GB SATA
O/S: Windows 10

So, everything has been running perfectly for the last few years with the default settings in the BIOS. However, last night I decided to play around with the o/c settings in the BIOS.

Using the "Load Optimised CPU OC Setting" in the BIOS, I changed the setting to "Turbo 4.4GHz". Windows booted fine and I ran it through PRIME95 for a few cycles. Everything seemed fine, but the temps were sitting at about 82 degrees C.

I rebooted and changed the "CPU Core Voltage" mode to "Fixed Mode" in the BIOS. I then changed the "Fixed Voltage", but instead of decreasing the voltage, I think I *may* have increased it... I now suspect that I might have set it to something like 1.400V.

I saved and exited and the computer wouldn't boot. It sat there with a black display. Unable to get back into the BIOS, I used the clear CMOS switch on the motherboard and reset it.

On the plus side, I'm now able to get back into the BIOS. Since resetting and with the default BIOS options loaded, the BIOS reports the CPU is sitting at a comfortable 38.5 degrees C.

But, on saving and exiting the BIOS, the PC goes into an infinite loop. It displays the POST screen, then reboots, over and over.

To test the machine, I have disabled all the HDD's and the SATA controller. I am using a USB bootable version of Porteus.

With CPU Ratio set to "Auto" mode
CPU Core Voltage: Auto
Outcome: Power on, POST, power off, repeat.

CPU Core Voltage: Fixed Mode
Fixed Voltage: 1.100V
Outcome: Proteus boots and works fine

With CPU Ratio set to "All Core" mode
CPU Ratio: 16 through to 20
CPU Core Volt: Auto
Outcome: Proteus boots

CPU Ratio: 21 through to 30
CPU Core Voltage: Auto
Outcome: Proteus starts to boot, then PC powers off and restarts. The higher the CPU Ratio, the sooner into the boot process the reboot happens. - Changing the CPU Core Voltage from Auto to a fixed voltage of 1.100V allows it to boot through any of these CPU ratio ranges.

Now, it would seem like the best thing to do is to leave the CPU Ratio in "Auto" mode and set the voltage to 1.100V. However, these results are inconsistent at best. I can occasionally set the BIOS like this and Windows will boot fine - Prime95 will even run for as long as I want it to, without any ill effects. Heck, I even managed to set the "Load Optimised CPU OC Setting" in the BIOS to "Turbo 4.2GHz" and run Windows for 30 minutes.

But, if I turn the computer off and on again, when it starts up, there's a good chance that the computer will reboot during the Windows boot process.

Soooo... a bit convoluted, but any ideas? I'm happy to retry and record any testing that anyone suggests.