Totally agree with hgeifman, I was a AT&T tech for 40 years, recently retired. You could have the best modem or router inside, but you have no idea how fragile the outside infrastructure is. Especially if you are still on copper cable. There is so many connection points between your router and the central office that can fail slowly over time, you would not believe .
Don't trust the test that are ran remotely from some office when you call in to complain, let them send someone out to test the sync at your router, and if there is a issue work their way back to the source.