Until last month, for 4 years I had used Comcast/Xfinity Residential Internet with my D-Link DIR-645 router for my home wifi without any problems. My wifi was stable and reliable and I didn't have to restart the router once due to wifi issues in the whole of those 4 years. Life was good.
Did you check and confirm that the built in wifi in the comcast gateway is disabled? If not, I'd start there. Possible both networks are interferring with each other.