AmeliaDu, I also am having this problem, I spoke with a Comcast tech and I was told the problem is probably because of my modem. I am using the Motorola surfboard SB5120 and that I needed to upgrade to the 3.0. Also another probable reason is my internet speed is fast and the unused Mbps are frying the modem. Do not see the validity in this since everything else works perfectly and I can stream movies from other providers. I cannot see the validity in this diagnosis, and shows it is an internal problem. Their solution was to rent me a new modem..LOL, nice tactic to get me to give more money, but was not going for it. Been reading these post and have yet to see a solution to the problem. If you are provided with a working solution I would like to hear it, as Comcast is not in a rush to provide customer service (except when it comes to signing you up), and forget about customer loyalty.
Good Luck,
Comcast Customer