Centurylink DSL, 20meg, Technicolor C2100T, Fedora

So every once in a while I click on a link and nothing happens for several seconds or streaming a video, buffering, and it got annoying. So I set up a testing script using speedtest-cli, MariaDB, and Phplot. I run speedtest-cli every 5 minutes and I'm surprised. On a regular basis, every 90 to 100 minutes my speed drops from 19Mb to less than 10Mb. And sometimes it drops to less than 1Mb. IE .2Mb and stays there. Centrurylink says I have too many DHCP reservations and to remove them :-\ or they want me to disconnect every device, turn off WiFi and run a speed test - Hey look it's 20Mb, there's no problem.

I don't have anything internal that runs on the schedule I'm seeing

Suggestions? Thoughts as to the cause. Thanks for any advice.

Each tic mark is 5 minutes. Red is download, green is upload