I too have the same problem of jerky video playback (on YouTube only) and I too changed nothing in the way of software or hardware that could have caused this. I have a high-speed (10 megabits per second) connection. I posted to YouTube's help forum over 2 months ago but since their initial response, they've ignored me. I followed their instructions (see below) but it didn't fix the problem. Any ideas as to what it could be and how it can be fixed would be greatly appreciated.
Liz YouTube support has posted an answer to the question "jerky video playback":
How odd! I suggest the following steps to try and optimize playback. Hopefully this helps videos play more smoothly again.
-- Upgrade to the most recent version / newest version of your web-browser (aka Firefox, Internet Explorer, Safari etc.).
-- Uninstall the current version of Adobe Flash player that you're running on your computer. Then, once you've uninstalled it, go to the Flash website and download & install the most recent version of the Flash player.
-- Temporarily disable any ad-blocking software you may have installed to see if this helps you play videos.
Thanks for visiting the Help Forum and posting your question! Good luck