Internet keeps on dropping connection or becoming really slow

Our internet has been dropping or becoming unbearably slow, to the point where we can’t even load a Netflix movie. We have been having this problem for 9 months. We have contacted customer service more than 20 times! Every time they run tests they say its healthy or fine but it’s like they don’t understand the word inconsistent, sometimes it works, most of the time it goes out and stays like that for 8 hours. They always end up sending a technician out but that doesn’t help at all, they changed all of the lines to our house and to our modem 5 times, still nothing.  We changed our modem about 30 times still nothing i work from home and it’s getting to the point where I’m going to lose my job and that’s truly unacceptable. I believe it has something to do with the lines in my neighborhood somewhere because it’s even happening at my grandmas up the street from us. Like I said I really need help here. We have been customers for over 10 years to xfinity and I really don’t want to have to switch service, but if it keeps on affecting my job then I will have no choice.