Why are my internet speeds constantly fluctuating on both wired and wireless connections?

I have the 1200 mbps plan from Xfinity. Until recently I was using their X7 gateway, but was having issues with internet speeds fluctuating. Sometimes I was getting 1gig, other times 200mbps. This was happening on both wired and wireless connections. I had a tech come out and he gave me new X8 gateway to try. Same problem, speeds were great one minute and slow the next.

I then decided to get my own gear; modem, routers, switches, etc. All quality stuff. I'm geared for up to 2.5gig throughout my home. Same issues though. At times I can get 1,200+ mbps on wired. My wife got 1,175 on wifi on her new iPhone 15. Other times, I'm down below 300mbps. These swings happen constantly throughout the day.

I know Wi-Fi can fluctuate so I'm not focused on that, but I experience the issue even when directly wired to router. I've used different routers, different modems, different PCs, and every one has the same problem with frequent and drastic fluctuations. I even replaced coax line that goes from my main splitter to my modem (one two-way splitter after wire comes in from street, one end of cable goes direct to modem, and the other to another splitter that feeds the rest of my house). 

Why is this happening? Maybe bad wire somewhere between the street and the first splitter? Any ideas are appreciated!