This is a compilation of ideas from the internet -
The most common cause of buffering is an internet connection that is too slow.
Note that even a broadband connection can have periods when it is slow due to increased traffic in your locality.
Another possible contributor is activity taking place on your PC.
Whenever your PC's processor is called upon to handle a task other than streaming playback, an interruption is possible.
Results will vary depending on numerous factors like processor speed and memory.
If streaming works fine as long as you don't use the machine for anything else, this may be the problem.
Occasionally this can be corrected by closing the Media Player and re-launching the stream.
You could also try going into your options and increasing the streaming media buffer.
Finally make sure your system is free of spyware.