Some Sleuthing Expertise, Please

[Axel_Lesch]

Hi @John_V
If your NUC runs in a case with a fan, it is most likely this:

Troubleshooting Overheating or Thermal Issues on Intel® NUC https://www.intel.com/content/www/us/en/support/articles/000033327/intel-nuc.html

Please have a look here

Rock running on NUC crashing nightly

If you haven’t cleaned the fan assembly, I’d do that now. Thermal cut-off will turn off the NUC to avoid damage, so this is a possibly / likely cause. iFixit have videos explaining the steps needed to remove and clean the fan.