My internet goes in and out speeds up to 700 then nothing

I have had this happen for the past year, I have called at least 30 times, had 2 technictions at my house, had the router replaced. I have constantly tole them its not in my house but the signel coming into my area, they will not belive me. I am so frustrates, belive me folk its not a rumer that if too many people access internet at the same time in the same area, the signel will slow down and sonetimes stop. Xfinity will not fix it. happens almost everyday. my advice to people is to find another service provider, if I could get a deffrent one in my area I would.