Have you updated the firmware on your router? That fixes a lot of problems. It doesn't matter if it is new or not, they are always trying to make their product work optimal for you.
Make sure your firewall is letting every app through that you need on the internet.
Call Comcast and have them ping your modem. They should be able to tell you if your modem is having issues and needs to be replaced or if it is the router that is the problem. I did the above with Optonline and they were spot on. The modem had to be replaced and I did that, the problem still was at issue, so they pinged the modem again and it was ok but the router was having issues. I replace it and all has been well ever since.
There is always the possibility you have a virus playing with you. Update your anti-virus and do the long run. Get Malwarebytes, install it, update it and run it.http://www.malwarebytes.org/
Take the free one, it works just fine.