My parents were having issues with their internet. I kept telling them it was ISP, butthey kept calling me instead. I went one day and went through everything in front of my Dad so he could see all I was doing. They had already called ISP 2 or 3 times as well, with all they ahd done, there was no improvement, I had him call the again. This time they sent a young kid who had just been there a couple months. Anyway, to make a long story short, the kid found the problem, it was the main feed line to the node for the area. It had a hard kink in the line. He climbed up and straightened it out, and no problems since.
I said all that, just to wish you good luck, if it's on your ISP side.