I'm having the exact same issue that people are experiencing here - the box isn't that old and we upgraded six months ago or so. The tech (after doing all of the reset stuff-unplugging, sending signals, etc.) is sending a new wifi cable box but it's not the wifi cable box as it's basically new-ish, it's some setting somewhere that is causing it. 

See this "closed" thread -https://forums.xfinity.com/conversations/new?categoryId=602d56f9e6b3965d18e3a594 (This is exactly what I'm dealing with...)