As a couple of people have mentioned, cables can also be a problem.
For the speeds you are paying for, you probably want Cat7 ethernet cable, not over about 50 feet, from your modem to your router. Another thing people don't think about much is the coaxial cable from the street to your modem. If there is a defect in the jacket, and it leaks a little bit, you get water in your coax, and dang poor signal.
