A possible cause of the problem is that something nasty is hogging most of your processing power. Download, install and run the free version of Malwarebyte's Anti-Malware:
http://www.malwarebyt...cts/malwarebytes_free
However the usual cause of the problem is simply that your internet connection isn't up to the job. Take a speed test here:
http://www.speedtest.net/
(NB: Click 'Begin Test', not the advertising link 'Start Now')
If you're not getting at least 2Mbps (and preferably at least 5Mbps) then it's unsurprising that you're experiencing buffering.
Chris