I am a service tech for a cable company, I deal with this everyday. You always have to start troubleshooting with the source. So call the cable company and have them check their stuff. Be aware that they may charge you a service charge if they deem that it is not the provider issue.
With what you are saying, I don't think it is a provider issue. I would agree with the others that it is your router. I had the same problem. Changed my router and no more problems.
Hope this helps. Good luck!