Definitely sounds like a "HW" issue to me... I would check:
1. Disable overclocking of any components if you previously configured overclocking settings. Reset the BIOS to factory defaults.
2. Ensure you are using the OEM power supply
3. Ensure you have adequate air flow around your NUC. I assume you can hear the blower moving air inside it?
If all else fails, and it's still under warranty, ask Intel to replace it.
Indeed I can hear the air flow, and I've not been overclocking anything so far and all bios setting are at factory deffault.
Well I bought my NUC second hand and when I enter the serial number It's written that teh warranty is over since 1900....
Anyway normally this hardware is under warranty for three years and is not out since three years, so i really hope they can help.