Since this is a technical question and I know the answer, I'll respond to this one (I have the option whether or not to see your posts):

If you're doing something else while watching it, then it means that you shouldn't be.

If it happens in the exact same spot every time and looks exactly the same every time, then it's a problem with the video encode.

If it happens randomly, and you can go back and watch the same section that it happened to later on and it looks fine, and you have no other programs running while you're watching it, it means your computer isn't fast enough (either you don't have enough RAM, or your processor isn't fast enough - probably RAM.)