Let's not forget one of the most common issues: your ISP. When you rent a modem from them, you usually end up with crappy, used hardware. Additionally, they provide the line to your home that gets you the internet, whether by phone, fiber, or cable. That line has to go outdoors where you'll find bugs, wild animals, hostile weather conditions, and more fun stuff. I've had a number of strange issues, including a gopher dig up and eat through a cable line. I've also had normal ones where equipment failed or a technician did a poor job of connecting the line to my building. Lots can happen, so if you've ruled everything else you should call your ISP.