Best answer by AjTrek1
Glad to see things sorted out for you. If your Wi-Fi is fluctuating that's a good sign that there's an update required or the unit is failing.
I agree that having your own router and/or modem is always IMO better than using that provided by your ISP. Actually you may not need to buy a modem especially if the ISP gear is a combo router/modem. I use Google Fiber as my ISP but I tuned the router portion of the unit off and installed my own which is an ASUS AC5300. I did the same when I had DirecTV.
One last point...although the fix I suggested seems to have resolved your issues...that's not to say that the issue may not reoccur. However, as you said Spectrum may be throttling your bandwidth or you may want to invest in your own router as well.
The another option to negate poor Wi-Fi coverage is to put your Sonos into wired mode (Boost) and use the SonosNet. You get much better coverage. Click the link for more information on setting Sonos to wired mode. https://support.sonos.com/s/article/3235?language=en_US
Feel free to post back if you have questions or need more assistance from the community