It may depend on how you are loading the videos. If you're using the YouTube video app that came with your device, delete or hide it off in a corner somewhere and reinstall the app from the Play Store. For some reason, the supplied YouTube icon on my Galaxy Tab 3 8.0 did exactly what you are describing. Uninstalling the Samsung supplied app and reinstalling it from the Play Store helped a great deal, and while it still buffers at times, it's not nearly as often nor as long. Give it a try, it won't cost a thing a while it might server problems or 'something' wrong in the ether, this has worked for others, including myself.
Cheers,
Mike