Is this on youtube? It simply means that your internet connection isn't downloading the film quickly enough to watch it in real time. A good tip is to start watchig, then press pause and wait a few minutes. Hopefully the buffer won't empty as quickly
Is this problem on Freeview? Try retuning. My freeview stuttered on bbc 1 and 2. Now cannot get them at all even after retune. Other channels ok. Luckily I also have cable.
on the BBC i player there is a box at the bottom to allow lower bandwidth ,or something like that, just click on it ,cured the issues i had with it freezing.
when you start watching a programme ( i usually catch it when its doing the roundy roundy coloured thing), at the bottom of the screen it says 'use lower bandwith' the default setting is normal bandwidth....its there honest !!
I found out after a couple of weeks of getting the hump by clicking on the help button, explained it all...