having the same exact problem for the last few days. everything was fine last week, now every 10 or 12 seconds the stream stops, pauses for 10 of 15 seconds, and picks up at a random spot.
everything else works, it's definitely not a computer issue. the problem exists on the xfinity site across all browsers, while other video streaming sites work perfectly fine.
hope this gets fixed soon, the choppiness and constant pauses make the service pointless.