@lewis_F, the description of your issue appears to be related to a “limited connectivity” scenario and probably affected by interference of solid objects in the 5 GHz band.
I suggest you verifying your network configuration: IP address, DNS, wireless security/encryption. Sometimes the firewall or antivirus may cause this type of issue.
Please make sure you have the latest BIOS for your Intel® NUC, a new power saving setting has been included in order to improve the connectivity with this adapter.
I do not recommend replacing the adapter with the Intel® Centrino Ultimate-N 6300 because it is not a validated model for Intel® NUCs, I suggest you using any of the other compatible adapters or contacting Intel support for further assistance on this matter.