Have you tried rebooting the tablet? Also, you should make sure there isn't anything else running in the background when you are trying to watch the Videos. The more that is being run on the tablet, and the lower the resources get, the more issues you will have.
You should also check your access. If it is through a data plan, then you may be getting issues when you reach an amount limit as your carrier/service provider may be slowing down your access speed. The same could also be happening with a WiFi connection. And if there are other devices on the same WiFi that can also give you problems.