Oopsie,
I’d also say it’s unlikely that replacing the modem multiple times would result in this same behavior if it were just the modem. I have seen it happen in the past, but it’s very rare to get that unlucky.
If multiple modem replacements didn’t fix it, it’s quite possible what you’re running into is an issue with the wires in your walls, or the wires between yourself and the box down the street, or the box down the street are malfunctioning. You’ll probably need them to send someone to your house to test that hardware, or see if a tier 2/3 can run longer duration test back from the ISP to your hardware and see the drop on their end. The trick is figuring out which hardware is the culprit, and these tests can have 3-4 possible culprits for each hop that has problems.