Many reason to freeze videos. The main reasons are 1) slow speed internet connection, 2) Poor graphic cards, 3)Running so many programs at the same time, 4) Some times so many people viewing the video at the same time.
A number of factors could be causing this. The most likely is that your computer simply can not handle the load. In this case, get either a new computer or purchase more RAM and a better graphical processing unit (GPU) for your current one.