Severe Slowdown on Phoenix Premier Level

I have an FCC provided "SamKnows" speed monitor installed on my cable modem connection that makes daily measurements of up/down speeds. I have a Motorola SBG6580 Docsis 3 cable modem. For most of last year I observed the doubling of my average speed from 50 to 100 Mbps, but at the end of November the speed gradually slipped back down to below 60 Mbps. See graph below.

I have tried several things like rebooting the cable modem and router but there is no change. I have used speedtest.net to spot check and confirm this poor performance. This is an unacceptable level of performance when compared to the consistent 100+ Mbps before the end of November.