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Thank you for contacting Intel Communities.
I am sorry to hear you are having power issues with your Intel® NUC. Can you please provide the following information to better assist you?
- BIOS version: (4 digits after "86A")
- Have you tested a different power adapter?
- Did this start happening after any software/hardware update/change?
I have to agree with Juan, this feels like a failing (intermittent) power supply. Good news is that replacements are available fairly cheaply (use search "DN2820FYKH power supply" on Amazon, for example).
Note: If you were seeing spontaneous reboots, as opposed to power loss situations, it could also be as a result of a failing SODIMM.
Hope this helps,