I used to have that kind of trouble. Constantly losing the signal would scramble the dsl router making me think the router was bad. My isp was able to monitor signal quality to my house. I worked with them by taking all line phones off service, moving the dsl router to different jacks in the house etc. That gave them what they needed to narrow down the cause.

It was finally identified the phone line to the house was poor. The phone company responded by slowing the dsl service down some. They said they could do that because they were still providing the speed I was paying for. I responded by going to cable based internet and dropped the home phone at the same time.

So, it could be a bad connection at a phone system switch somewhere, or it could be a poor ground, or weak insulation before the line comes into the house. Even a bad line phone or filter inside the house can be involved.