Do you have a splitter on one outlet feeding TV & modem or modem & phone? I had a similar issue at my previous apartment and it turned out after 4 years the splitter that Comcast installed on my outlet for TV & modem failed. Finally a guy from tech support told me to try a new splitter, just get a basic one at Radio Shack. He said they go bad all the time and it can mess with the signal enough to make your modem not work (it needs higher bandwidth) but the TV still work.