I guess you mean youtube!? The problem is likely due to your computer's processor being too slow to download the video smoothly, or your computer is too full to work efficiently. Do these:
1. empty the recycle bin to remove dleted files - recycle bin, empty the bin;
2. delete the temporary internet files/browsing history - Tools, internet options, delete;
3. Do a disk cleanup to get rid of unecessary files - (depending on your windows view) its either
i) start menu, control panel, performance info, open disk cleanup, all files, ok.
ii) start menu, control panel, system and main