Have you seen any behavior from your computer that indicates that you may have a virus or malware? Either of those things could cause that kind of problem. Things like browser tabs or windows opening by themselves, strange search results, unexplained browser crashes? I don't know what anti-virus you use but have you tried doing an in depth or detailed full system virus scan just to check? Have you tried using some free software like Spybot Search & Destroy or Malwarebytes to scan for things that most anti-virus software doesn't look for?
There's also a chance that it's not your computer or software, I can't find the thread but someone else who reported this kind of thing finally had a Comcast technician out and was told "Your signal strength is very low" and the tech replaced the wire from the pole to the house and the problem was solved.
- Norm