You could try
Clearing your browser cache and cookies.
Loading a You Tube video Right click on it Click on settings If hardware acceleration is enabled then uncheck it.
You can change the default buffer settings in Windows Media Player, use instructions below: 1.Start Windows Media Player 2.Click the View 3.Click Options 4.On the Advanced tab, click Streaming Media(Windows Media) 5.Click Change to alter the buffer settings.
You should also make sure that your network drivers are up to date.
If you log into your YouTube account there should be an option to enable the "I have a slow connection" option.
In BBC iPlayer are you using the lower bandwidth option which is at the bottom left of the screen?