Sounds like your bandwidth is insufficient for videos -- either your wireless signal (if you are using wireless) is poor, or your isp is not providing you a sufficient connection.
Try attaching to your Internet router by a wired connection to see if it helps.
You can run SPEEDTEST
top check your connections speed, and inSSIDer
to look at your wireless network signal.
You might also try using Speedtest with your computer wireless and then wired and compare to see if your wireless is slowing you down.