Weird network dropouts occur repeatedly on Windows 8.1 (Intel NUC)

I am using an Intel NUC mini PC here. It is ruing Windows 8.1 x64 and is used as a media center and communication PC. I want to also use it to access my media accross the inteet using JRiver MC which is configured accordingly.

I am NOT using any WLAN or other non-wired network here. Everything is coected using cables and the network including all hardware is 1000 Base-T everywhere.

My inteet provider is Unitymedia Germany and the router is a cable bound Fritzbox 6490.

On the NUC I also use Virtual Here (see https://www.virtualhere.com/) to coect several USB network devices in my LAN to the NUC. FYI: Virtual Here uses Bonjour to detect its servers.

The problem is that the network on my NUC drops out every few minutes (sometimes even every few seconds!) and after about 5-10 seconds of being unreachable it's up again. The problem seems to get worse under heavier network load e.g. when I try to stream a video across the inteet.

I can assure that other computers in my LAN do NOT show this weird behavior. I have a Synology Disk Station here, too. It can be reached across the inteet non-stop and copying even large files is NEVER interrrupted at all even if the copy process takes hours. The NUC is the only computer here which loses his coection repeatedly every few minutes.

I found other people across the inteet having similar issues but all the solutions posted there were not helping my issue.

The way it is now I caot watch any streaming media unsing JRiver MC when I am on the road because of these dropouts. After a pretty short while the coection is lost, the player stops. Sure the retry after a short wait is successful but it happens over and over again every few minutes or sometimes even after a few seconds. It's simply unusable that way!

So what shall I do now?

Btw: The NUC has been replaced two times and it makes no difference. So either it's a systemic problem with the Intel NUC OR (and this is what I believe) it has to be a software configuration issue or software conflicting with each other etc. But I can't find the reason.

So far I have done this:

• uninstalled and reinstalled Windows and everything else
• uninstalled + reinstalled the network driver
• reset winsock
• disabled IPv6
• replaced associated network cables and switches
• replaces the NUC with another NUC two times

Nothing helped so far. Any ideas? Shall I try Wireshark to detect the issues so see what happens here? But what would I have to look for??

Thanks.