Choppy Youtube viewing.
Did you install a Beta version of Adobe Flash Player or did you install it from adobe.com?
If a beta version has been installed, uninstall this version and download the latest version from Adobe website.
If this step does not help, you can also try to update the video drivers. For on-board graphics, refer to this link.
For discrete graphics, refer to this link.
Follow the on-screen instructions to complete the installation. Reboot PC.
Here is an article on Adobe forums about Video Playback issues. The bug fix report can be helpful.
Hope this helps.
Please reply if you have any further questions.
Thanks and Regards,