If the latest BIOS does not control computer temperature, it might be the processor fan.
Intel Customer Support may replace the fan and heatsink or replace the NUC.
My recommendation still is to contact Intel Customer Support for warranty replacement of your NUC. You can use our Web Ticketing system to do that.
Online warranty:Service Request Email
Chat support: http://intel.com/chat
So I have a problem with cooling system and 50 °C - 122 °F in idle it's too much ? Another RMA (first was DOA) ?
Please people post your temperature in idle and in load and the monitoring software.
50C seems ok, the fact that your processor can go up to 105C means that is running under specifications.