Essentially you've replaced everything and the error is still occurring.
Low level things like memtest and BIOS isn't the best of signs, however as mentioned a faulty HDD can cause these errors.
For instance I have not long bought a HDD that needed a firmware flash update, it also caused a lot of errors with my computer because they didn't communicate properly.
A faulty HDD can cause random errors and since you have replaced the likely culprits you should disconnect all hard drives then try snooping around the BIOS or boot into Windows or Linux off a removable media.
Your CPU utilisation ups when you are in the BIOS, without any hard drives or any connected media that could cause problems you'll see whats up.
If the problem occurs still try disabling all but the default cores for your CPU. Leaving only the default core will allow you to see if the other cores are in fact dodgy.
A CPU with a dodgy core can still boot up and POST in some cases, just like when you reactivate a disabled core on some processors.
So yeah, try disconnecting all devices (hard drives), test again.
Then try disabling cores and take note of temperatures voltages and of course fatal errors.