While the "15th Gen" NUCs are technically the newest era of NUCs (now handled by ASUS), they typically use the Intel I226-V or I225-V 2.5GbE controllers, which have a well-documented history of "link flaps"—randomly dropping the connection for a few seconds before reconnecting. The Intel I226-V and its predecessor (I225-V) are known for a hardware-level "energy efficiency" bug that causes the NIC to momentarily lose sync with the switch or router. This often appears in Windows Event Viewer as Event ID 27 ("Network link is disconnected").
Disable "Energy Efficient Ethernet" (Most Effective)
This is the primary culprit. The NIC tries to save power during micro-seconds of inactivity and fails to "wake up" fast enough, dropping the link.
1. Device Manager.
2. Expand Network adapters.
3. Right-click Intel(R) Ethernet Controller I226-V (or I225-V) and select Properties.
4. Click on Power Management tab.
5. Uncheck "Allow the computer to turn off this device to save power".
6. Go to the Advanced tab.
7. Find Energy Efficient Ethernet in the list and set it to Off (or Disabled).
8. Find Ultra Low Power Mode (if present) and set it to Disabled.
9. Click OK.