Does it do this regardless of the software playing the video e.g. YouTube and VLC or WMP or XMBC or whatever you use to play your videos?
It happens on Youtube and in Windows Media Player. Quicktime, oddly enough, isn't playing any videos at all; I never actually used it for anything before. (This may be a codec issue. I'll fiddle and see if I can get it to work.)
Update: Apparently, Quicktime for Windows is incompatible with Divx/Xvid codecs, which is why I can't play my .avi files in the Quicktime Player. There is a codec called "3ivx" that is supposed to work, but the creators charge for it.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous thread is at close to 500 comments.