Can you please provide some more information? While you describe the problem "EVERY video I watch freezes, stutters, pixelates, looks likes like everything is happening in slow motion, voices don't sync with what's on screen" you don't mention what web browser you are using and what operating system?
A few generic suggestions, if you are using IE on anything less than Windows 8 then check and make sure you are using the latest version of Flash, you can check that here: http://www.adobe.com/software/flash/about/ there also have been complaints and an official statement from Comcast that a reccent Windows patch is causing problems between Flash and IE and that Adobe is working on a fix; that problem only affects IE and not any other browser.
If you are using IE, many people have better luck with a different browser so give FireFox or Chrome a try.
- Norm