As given you have some hundreds of possible causes. The most common I run into today is an overreaching security suite. Also, be sure it's not browser related. Always test in another browser.
PC running Windows 7 ultimate. Lately I get video buffering all the time and cannot see short videos. On speed test to my ISP, download is fine but upload starts well then drops to practically zero and the internet connection is lost. What could be happening? A laptop connected to the same router works fine.