Greetings,
When a system experiences boot failure with a message such as yours... I would start with the obvious. Weak or dying MB battery, BIOS corruption, weak or dying PSU and/or failing system component. Since the only things we do know are:
You have this error intermittently from a cold boot, and
Z170x-K3 (Rev 1.x?), BIOS F1 and i-6700k with an undisclosed amount of RAM running at 2133Mhz... I'm merely making suggestions based off the limited info you are providing.
MB batteries should last more than a few years, but sometimes they don't and a weak one can yield unexpected and inconsistent system behavior. They cost about .50c and its worth it to rule it out as a problem.
A weak or failing PSU can cause erratic boot behavior because this is one of the times where it is under the most load, just like the battery in your car when starting.
BIOS corruption when not severe can also manifest itself causing intermittent boot failure. Not saying this is your issue, but its easy to check and rule out. I'd be remiss not suggesting it.
A failing system component if it doesn't respond during power on self test or boot sequence can as well, cause a boot failure. Sometimes it works, sometimes it doesn't.
There is no reason to be fearful of resetting your BIOS, and in some cases corruption that has occurred can be resolved by resetting. Set Up defaults also provide stability and safe configuration when troubleshooting. Much has changed in 9 yrs.
The exception would be UEFI RAID installed on m.2, but this board only has one m.2 connector so Legacy RAID on SSDs is not affected by resetting the BIOS as it can be easily recovered.
How much information you want to share about your system is up to you. I've provided basic troubleshooting steps to help guide you towards a solution. Maybe it does have to do with your BIOS settings and how you installed your OS, but we don't know.
Cheers