Is your DSL line the same as the phone line? Is the line clear when you make a regular phone call?
What you describe is like when ants built their nest in my closest termination box. The data would stream intermittently, and I couldn’t figure out why I had problems. But when I took a phone call, and couldn’t understand person I was talking to because of all the extraneous noise, I called the provider, and they evicted the ants. Everything cleared up after that.
I had intermittent connections a few months back. It was caused by the ignorant woman across the street constantly backing her huge 4X4 truck over the junction box. She was using the box to tell her when she got close to the fence and had pulled my connection lose in the process. The Internet would work, sort of, until it rained. I’d cal GTA would come repair the box and the next week I’d have to call them again
The telephone company finally put up a concrete post in front of the junction box