For a video to play without pausing it needs two things:
First: Quick download from the web. For an internet video to play on your computer it needs to download the file to your computer.
With some sites (like Youtube) the video starts playing even if it only has part of the file on your computer BUT if the web site is busy, or if your broadband link is slow, then it may be that the video "plays" quicker than it can be downloaded.
So you will get delays while it waits for the next bit to download.
Secondly, your computers needs to "play" the video, but if your computer is doing other things then it will allocate a bit of time to the video and a bit of time to the other things.
So it may be that the video is pausing waiting for its allotted time slot.
My guess it is the first problem, that the video is playing quicker than it can be downloaded from the internet.
This can be if the web site, or your broadband, or the web in general, are all busy.