Thanks All - New Router, CAT-8 Cables, Grounding Boxes, and THC - Got It!
These are all great ideas that will likely improve sound quality (esp THC - what is that, “Total Harmonic Control” or something? Sounds promising.
But none of these will fix the dips in quality, which in case you haven’t read my post above, I agree with everyone that has said it’s not my network. Because, playing an equivalent local file is also bad when quality is bad.
That leaves noisy AC power and RF as likely culprits, but probably AC. And as above I’ve tried many conditioners and isolation transformers. It may be that my only solution is a Stromtank, one day down the road.