here's what it usually is:
1. something in the line. Lines splitter, attenuator, amplifier, kinked cable, bad cable, moisture in cable, corrosion to connector.
2. The line from your provider is bad.
Usually the best bet is to directly connect the line wherever it enters your house to the modem. Most isp's won't do anything once the line enters the home but they are responsible for it prior to that point. By directly connecting it, it removes the chance that something internal to your house is causing the issue. If its directly connected from the line coming in and you're still getting these issues, its an ISP issue that they need to address either at the ISP box or line running to the home.
