Possible cause 1: There's a problem with the server sending you the video file. (Likelihood: Effectively nil, if you're experiencing the same problem with several different sites).
Possible cause 2: There's a problem on the 'backbone' of the internet, slowing everything up. (Likelihood: Almost nil. The internet is sometimes slowed by infrastructure failures or massive DNS attacks but, if so, everyone would be complaining about it).
Possible cause 3: There's something nasty running on your PC which is hogging nearly all of the memory and causing problems with buffering. (Likelihood: Moderate to high. Some spyware programs can cause 'memory leaks). Run your anti-spyware program(s), such as Adaware or Spybot to look for possible infections.
Possible cause 4 (a variation of cause 3): There might be some legitimate software running on your PC which is slowing things down. (Likelihood: Depends on what you've got. If you're using Norton or McAfee, it might be worth temporarily disabling your firewall to see if it's the cause of the problem. If so, remove your existing firewall and install the free version of either ZoneAlarm or Comodo). Also try pressing Ctrl-Alt-Delete, entering the Task Manager and closing all applications you don't really need.
Possible cause 5: The speed of your internet connection could have dropped to an unacceptable level. (Likelihood: Moderate to high). Check the speed here:
http://us.mcafee.com/root/speedometer/default. asp
If the result is low, contact your ISP.
Chris