I uploaded this clip to test out my new lens, and had loads of screen tearing going on.

YouTube - Wide ANGL :) (http://uk.youtube.com/watch?v=nFKXRmucFds)

Its the raw clip straight from my camera so was filmed 1080 50i. It's not interlacing thats causing those lines though. Is this just something to do with having to many tabs open on firefox or something ?

I can say for sure that I see no lines at all. Have you seen it on other computers?
Tearing can happen because of your machine, and it can happen because of the program that it's playing off of -but that probably isn't the problem because the video really is fine on my end.
Screen tearing - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Screen_tearing)

K thats brilliant, thanks for your help once again aric !

I see the screen tears. It's not that noticable because you don't have anything shifting quickly on the horizontal axis. I recently posted a topic similar to this here http://www.dvinfo.net/conf/dvd-web-video-delivery/140176-strange-lines-problem.html

Here's a video that was made by panning a photo in After effects. Photo panned from AE on Vimeo (http://www.vimeo.com/2614032) I see occasional screen tearing in the logo. Do you notice the tearing in this video if you watch it a few times?

I looked into this quite a bit and see that it is fairly common when playing games at high refresh rates on monitors that have a fixed refresh of 60hz. I think since video is "refreshing" at 24 or 25 fps and the monitor at 60hz this mis-sync may have a lot to do with this. What I need to understand is why some see the screen tearing and others like Aric are blessed. If you resolve this problem please post the resolution here.