I know that it may not be easy to believe, but most of Comcast's network problems are in the home. For example, my home was wired by my alarm company over 20 years ago. They included many splitters, without amplifiers. The result was horrible power level throughout the home, and odd behavior at set-top boxes, and at my cable modem. The advice given on this thread: make sure connections are tight, etc. is sound.
If you find that the connections are tight, and you see nothing obviously wrong, then perhaps having a technician visit your home to check power levels is yourbest option.
Now that I have typed this, I see that your question is over a year old. Sorry. I'll post my answer, anyway...
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