As much as I dislike Comcast, I voted you up because I once also had this magic serenity for a few years.
Then I switched to Comcast for TV and it all went down the tubes. Random outages (sometimes daily), substandard speeds, Netflix speeds suffering (or stuttering) at peak times, etc.
Who knows whether this just reversion to the mean, or the techs botching the install. Even "reddit trick to email the Comcast VP" didn't fix it (though that did get a quick response).
