Sorry for the late reply, but rereading your post reminded me of similar issues I had a couple years ago. In my case it turned out the short ethernet cable I was using had an older cat-5 data rating, so I replaced it with a newer cat-6 and haven't looked back.Might be getting this, if at a lower price.
My current 2nd gen 4k has no issues streaming, but the UI is quite laggy when switching between apps, and it only holds 2 apps in RAM, when u switch to the app I used 2 apps ago, that app is being reloaded and I have to go find the show that I was watching... it's an annoying behavior
In the process of figuring that out I came across an app called TV info that lets you see how much ram and storage is being used. Sometimes a quick restart is needed to clear ram. Of course closing and/or offloading unused apps also.