Problems with new Dell Precision m5520, monitor, LAN, video

I have problems with ethernet randomly switching on and off. The cable is connected to the dock. I have the power saving thing swiched off from the device manager menu.

Another problem is with the external screen that I use working on the office, I have my taskbar there and it constantly flickers. The icons just disappear and come back randomly. The screens are different resolution (4K and 3K) which also does some annoying scaling issues sometimes.

Third problem is with videos over internet, they are laggin really bad. I would think that there is enough power in this 3500€ machine to run them.

I installed all the drivers, BIOS update, windows updates.

products...

DELL m5520 i7HQ/15.6UHT/16GB/512SSD/M1200

DELL 27 ULTRASHARP U2717D QHD

DELL WD15 DOCK 180W EU

Thanks