Just got back from my folks' house, where Mom and Dad were each encountering the same problem with their respective computers: frequent, sporadic losses of Internet connectivity.
Basically, they'd be connected one minute, then getting Page Not Found errors the next.
Sherlock Holmes cap firmly in place, I set out to pin down the problem. Though I usually start by pointing the finger at Windows, I tried getting online with my own laptop and encountered the same result. No way could three computers be suffering from the same Windows glitch. Thus, the problem had to lie elsewhere. (Elementary, my dear Watson!)
In cases like these, my first fix is always to power-cycle the modem and router. But Dad had already tried that, and I had no better luck.
One possibility was the ISP, in this case the cable company. But in my experience, service outages aren't like someone flicking a light switch on and off. The connection goes down for 10 minutes or an hour or whatever, then comes back.
More likely, something was wrong with either the router or the cable modem. Again I turned to the process of elimination: I took the router out of the equation, plugging one of the machines directly into the cable modem.
Success! No more on/off Internet. The culprit was definitely the router. Who knows why it hopped the fast train to Busted Town--it just did. (For the record, I did try updating the router's very outdated firmware, fully expecting that that would solve the problem. It didn't.)
The solution: a replacement router. The outcome: happy parents. The case: closed. I hope you'll keep this in mind the next time you're troubleshooting a flaky Internet connection. The process of elimination is always the best way of figuring out what's causing it.