A couple of questions and suggestions.
Please provide your full system specs. All hardware, memory, cpu, etc.
Are you overclocking, mem or CPU? Your last sentence states everything is at stock, but wanted to confirm.
Please review page 23 of your manual, and become familiar with the SB and BIOS SW. One tells the board which BIOS to start from, the other isolates the BIOS' from one another.
Start with this. Disconnect power to the PSU. Perform a battery pull. Wait a few minutes. Re-insert the battery and reconnect power. Power up, enter BIOS. Configure only what is needed to boot/run the system conservatively. F10 to save and system will restart. Get a clean boot??
If yes, shut down gracefully after OS loads. Refer to the manual, page 23. Move the SB switch to position 2 "Single BIOS". Perform a cold boot. Does this work?
If the above fails, power off leave SB switch set to single BIOS and switch the BIOS switch to boot from the other BIOS. Cold boot, DEL to enter BIOS and make the same settings to boot/run conservatively. F10 save. System will restart, should boot into OS. If good. Shut down gracefully. Wait a few seconds and perform a cold boot. If successful, there may be a problem with whichever BIOS the system was unable to cold boot from correctly.
If neither are consistently successful, I think it would be good to give us the opportunity to review your hardware specs. It would be helpful to know more about the water setup and what headers you are plugging things in to.
At that point, flashing up might be recommended. Again, you'll only be doing one BIOS so this mitigates risk if you isolate as I suggested above. Further, flashing to F8... you will be using efiflash which is a DOS based utility and is about as "safe" as you can get performing a BIOS upgrade. Always some risk, but again less using this utility.
Good luck with the testing. Let us know how things work out.
For reference efiflash guide.
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