Usually video and sound errors are due to bufferring errors. I.e. you network/computer cannot download/process the sound and video quick enough.
pop onto youtube and have a look at any of the videos on there, check to see how they run, and what quality they are. 240 320 480 720 etc... see what is a comfortable rate to use. (The cog selects available qualities).
If these run ok, possibly it's the site you are running the videos from.
I would suggest more info as GB suggested - (if appropriate - post the link.)
A good start to check out your internet is
www.speedtest.net (select an appropriate server and check you connection)