weird playback moments

I didn't do anything to start this; it just happened. Recently, whenever I play a YouTube video - but not videos from other sources - on my computer, the video playback freezes every few seconds for just a moment before continuing. It's really annoying. It doesn't seem to affect the sound, and the freezes are frequent enough and short enough that the sound and visual don't get measurably out of synch. (This has nothing to do with slow file loading: the file can be completely loaded and it still does this.)

Any idea why this happens, and how to get it to stop?