I dont really know were i should post this, so if its in the wrong place i'm sorry, now onto the problem...
...For some reason some the picture on my some of my downloads has started to get really jerky, the sound and everything else is good, but the picture isnt as smooth looking as it was before.
So far it seems to only be effecting all my Smallville episodes and the latest episode of 24, can anyone help?

Try viewing them whilst LW is closed. If the problem doesn't exist then it sounds like LW is using up too many resources. This might be due to using the Resume/Find Sources function on too many files, etc.
If still have the problem then perhaps your codecs need updating or get a backup player like VLC: http://www.videolan.org/vlc/ (choose nearest mirror site)