Have you tried to play the UHD content without the Gbit adapter, using your box’s 100 Mbit port? Do you see any stuttering then?
Here is a thread for Working USB Wifi/Ethernet adapters for CoreELEC which could be helpful. Try to run som iperf3 tests with different settings to measure bandwidth, packet loss, delay jitter etc which could give us some insight why the content you want to play is buffering.