I often see very slow buffering on YouTube. But I have a feeling it's YouTube trying to be smart about not allowing video to buffer too far ahead of the client. Makes some sense, but can cause issues. I also see some videos that no matter what you do, they inexplicably buffer much more poorly than all the others, causing pauses. My guess is that this is caused by a connection (for that video only) to a particularly slow server.
The problem here is that it's so slow it can't keep up at all. I can't watch videos in 720p because the buffering isn't keeping up with the video bitrate. It was not like this in the past, and it's not a problem with my connection because other streaming sites will stream HD just fine. I have tried many different videos and they all have the same problem, even Gangnam Style can't keep up with 720p. I've eliminated it being an issue with the specific CDN endpoint by changing my DNS servers to ones in different locations, and using a VPN I have no problems. Something is very fishy here.
Also, to the OP, I used to get similar unsolicited calls from Comcast pretty frequently. It seems to just be a thing that they do. I stopped answering their calls and eventually they stopped calling me.