I figured out the problem. You need to delete what’s in your instant queue.
Long story short. Netflix “streams” not only the covers of your movies and TV shows, but also if you’ve watched them, like them, and so forth. The more movies and TV shows you have, the more Netflix is already using of your signal (to display them on the XBOX software).
If you have a lot of shows and movies in your queue, and press play on any movie, it should show a weak signal and start out (sometimes it doesn’t start at all) with horrible pic quality. But then 3 or so minutes into the streaming, your XBOX will likely jump back to a normal strong signal and you’ll be seeing your shows / movies in the best quality available.
Reason this happens is cause Netflix drops the “streaming” for all those video covers and the content within.
So keep your instant queue as empty as possible, and you’ll be fine.
Your internet signal is fine and the Netflix software is fine. I hope this helps!