Do you keep track of what type of codec is being used with the different video types? Have you changed any codecs on your system lately?
Do you know what the bandwidth of the video being streamed is?
One thing you might try is, if you can, when you find one misbehaving, download it completely then watch from your system to see if it does the same thing.
Many times on my system if I make the video full screen it looks bad or breaks up.