DSL. I'm pretty sure water gets in the lines. One of the techs told me about 2 or 3 years ago that the cables need to be replaced but the phone company probably won't do it because we are the only residence on this side of the road so it's not cost effective for them, and as far as I can tell so far he has been right.
But, another possibility is a bad connection coming off the pole. We had that problem in the 80's. Techs came out 3 different times but didn't get the problem fixed. The 4th tech they sent was a woman. She pulled her van up to the pole, got up there and fixed it in just a few minutes. I told her our trouble history. She said the problem could be seen from the ground (likely due to squirrels), but, "Those guys won't get out of the truck, they won't climb a pole. You want something done, you call a woman!" I liked her.
I don't know if squirrels are contributing or not but I think the lines must be getting wet somewhere.