A few things to check.
Is your Flash Player up to date? Have you tried unchecking the "hardware acceleration" in Flash settings? -Right click on the video when it is running, click on settings. You do not say which browser you are using - have you tried clearing the cache and cookies? On YouTube will your video buffer if you pause it whilst playing? If not will changing the quality by choosing a resolution from the gear icon help? Do you have a good broadband connection?