It is a common problem with DOCSIS! And has been the case since 1.0 era. DO NOT use Cable if you can.
It happens when it is over subscribed, as well as other radio interference. Although common cause is over subscription. You may actually try downgrading your speed / DOCSIS 2.0 / 3.0 to see if it helps.
Funny enough with G.Fast, had they continue the investment running 1Gbps on telephone cable would have still been better than DOCSIS.
I used to fight this before I got my fibre optic. Even 5G Internet is better than DOCSIS. For people who have never been through this, you will quickly learn the one thing most important with Internet or WiFi isn't Speed, bandwidth or latency. It is reliability! It is far better to have a reliable slow 6Mbps ADSL connect than a 1Gbps Cable that has connection constantly dropping off.
I dont know about US, but in most places on earth Government seems to be ok to mandate electricity and water as well as telephone line as standard. I wonder why we cant mandate Optical Fibre as standard as well. And it seems most property agency refuse to put internet connection speed and types on the property pages. If consumer could easily learn without optical fibre equals shitty property and refuse to buy or rent the land lord will have interest to quickly act upon it.