I, too, have been having this same quality issue since around November 11th (and I've posted here a couple times previously).
However, starting on Sunday, December 1st, my experience seems to have improved. That week (last week), starting on Sunday, I streamed from Netflix every single evening, at prime time, and every time the stream either locked in at Super HD right from the start and stayed there, or did so after just one or two stop/starts of the stream. Not perfect, but a huge improvement over what I was experiencing previously (and a situation I can live with, at least temporarily).
I was wracking my brain trying to think if there was anything I had done that could have impacted this. The one thing that stands out was that on Saturday, November 30th (the day before the improvement), my power was out about 45 minutes while I had electrical work done.
It could have just been a coicidence, but I wonder if a complete shut off, unplugged from the wall, for a solid amount of time may be helpful. It's worth trying.