Stuttering video in browser but fine played locally

Hi. I've put this here because I think it is possible a network thing rather than my computer. But I could be wrong.

I'm on a PC with XP spk3 and my broadband speed is 30+ Mb.

In any browser, if I go to Vimeo, Youtube etc and watch higher quality vids I get terrible stuttering of picture and sound, really bad crackling and warbling and drop outs.

But, if I download the same vids, they play fine on my computer.

I've updated everything I can think of and there is no improvement.

My machine is old and slow but this problem started just the other day and it was bad from the start. It didn't gradually creep up.

I'm at a loss. Can anybody make some suggestions?

I can email a small MP3 of the sound if it helps.

Thanks, Colin.