If you're using a DSL ISP connection, there are a number of things to look at. A DSL line in most areas requires a filter in-line, to avoid erratic, or interrupted, data streaming. Are you using a decent filter?
Since you use the term "modem/router", I guess that means you're using a one piece device, not a separate modem, and router. That makes it easier. Do you have another modem/router that you could substitute for yours, or borrow another one, to rule out a bad device?