Well, if you have a lot of other things running, or there are other applications/computers using the data or WiFi connection, or if your data or WiFi are being staggered, slowed, limited by the service provider, it will cause these issues.
First try making sure other things aren't running on the phone while you watch videos. Any other data being used by apps, or phone resources being used, will cause the video freezing.
Next, if you are connecting via the phones data plan, check your data information with your carrier/service provider. Do they lower the speed of it once you have hit a certain amount. Most do that unless you have a 'true' unlimited account. And those are really expensive.
Now check your WiFi speed and connection, if connecting via WiFi. Are others on the same WiFi? Is the carrier/service provider messing with your speeds (it happens). Are you using a company that limits how much data you can use for the WiFi before they lower the speed? Are you trying to access during peak hours? Usually 5pm to 12am.