This has been happening for months. Yes, I've reset the modem. And when I spoke with someone at Xfinity today, they told me that my modem wasn't too old.
Reading this thread, I see that other people have experienced the same problem and it turned out to be something outside their home, which I imagine could be the case with mine because my neighbors have been having the same problems for months as well. On that thread, the problem was resolved when they discovered a bird nest on the outside cables and for another person when they changed some other thing. I'm beyond frustrated!
Thanks in advance.