I would say with 95% certainty it is related to related to the hard drive, at least in my case.
I have been running with just a 128gb M.2 SSD for the last year+, never a single overheating problem.
I installed a seagate 2tb aptly named "firecuda" last week - and the overheating problems began the same day, which is what brought me to your post.
My system was also set to "Max Performance".
Running "sensors" on Linux reveals temperatures up to 92deg (celsius).
I have just made the above bios changes to the cooling system, will report back on the results....
Perhaps the hard drive produces adverse heat - or perhaps it is just the airflow restriction it creates?
I can confirm that:
a) Installation of a 2.5" hard drive into my system was what caused the overheating issues.
b) Changing my fan speed settings, as per N. Scott Pearson's suggestion, fixed the issue.
My operating temperature has dropped by about 20 degrees.
Mine is an NUC7i5BNH
Also, I can now play 4k videos in youtube in Linux/Chrome - something I was never able to before.
I was always able to play 4k with VLC, but not stream 4k from Youtube. Now I can. Thanks!
So clearly my system was on the cusp of overheating - then adding the hard drive, reducing the air flow, pushed it over the edge.
Changing the fan settings altered my operating temperature from about 95° to 85°, and made the system work.
But then I found the following post:
Following the tip here - reduced my operating temp from 85° to 37° (!!)
It turns out that, even though the NUC looks completely clean inside, if you remove the motherboard from the case, and then remove the fan from the top of the motherboard - you will find a bunch of dust trapped between the fan and the cooling fins / air intake.
It's non-obvious, because everything looks perfectly clean, until you remove the fan.
I can now restore my fan/bios settings to have a quiet NUC again.
Thankyou to user: STOKANATOR