Unreliable Internet Speeds: From 1300mbps to 300mbps.

For some time now, our home internet has proven very unreliable. The Xfinity app's speed tests seem to report that the internet speeds will vary on a moment to moment basis, where one minute it's a little above 1300mbps (the usual internet speed we had before this was an issue), but then suddenly drops out to 900, 700, or even lower than that. I have seen the number go down to double digits. The test itself sometimes 'freezes,' where it displays the same unchanging number for a few seconds or even the rest of the speed test, like it isn't able to update or receive information from the router.

Usually this is manageable enough, but at times where it drops down low enough it will boot us from things that require a constant internet connection, and more frequently causes stutters. The general trend seems to be a cycle between brief moments of high speed before slowing down dramatically, but never quite reaching zero.

As of right now, the speed test I am taking right now went from fluctuating around 500-700mbps before suddenly going back up to 1200. Funnily enough, because the internet speed test ended on ~1200, the app said that the internet was actually looking good. Needless to say, this assessment is wrong. If it's anything, the Xfinity AI assistant also apparently detected something strange going on about our internet, but I hesitated to bring a technician out here.

Restarting the router does not work, before anyone asks. On top of this, we finally got a fresh leased router and coax cable, however this has not changed anything. Apparently there have been a few instances of work being done in our area, but this has never seemed to do anything. For reference, our current plan is for 1100mbps.

Has anyone else had problems like this, and any ideas as to what it could be? What did you do to fix it? Should I even be using the Xfinity app's speed test for this?