Internet streaming depends on lot factors among them Internet Speeds, Source of the stream, popularity of the stream, day and time when you are trying to stream are worth mentioning.
You might have the fastest internet speed from your internet provider but it really depends how much traffic it is on the internet at that moment. The speeds tend to fluctuate according to the internet traffic. The Internet speeds are, generally speaking, slow on the weekends and during peak hours when everyone is trying to access the internet. When streaming a video from internet, the streaming also depends on the source of that video. If the video is from a reliable source like YouTube or Dailymotion or other media provider they stream fine, but if the video is from one of the torrent sites or pirated movie provider the stream may not play smoothly. If you are playing a video that is popular at that very moment, like a sporting event or a new movie then it might also buffer a lot as lot of people will be trying to watch that as well.
Buffering is pretty common, however it can be taken care of by carefully selection what you are trying to watch and who is the provider of that stream.