Does anyone know why this happens..

Digital Video does not do well with high contrast edges. Take a look at this tutorial for some great tips on creating text over video.

http://www.creativecow.net/cgi-bin/page_wrapper.cgi?forumid=3&page=/articles/hodgetts_philip/titles/index.html

Acutually i'm aware of the noise you'll get from a skinny font..I sent this video off for "captioning" and was returned to me and there were some "flickering" of the captioined letters..My client wants an explanation, i've told them that this is pretty much standard with white captioning over a changing background.

Their not happy with my explanation..

You could try applying a small amount of vertical blur to the captions and see if that helps.