Sony ericsson W850i screen goes blank?

Hi, I have a sony ericsson w850i and when i switch it on it will work fine, however if i continue to keep using the phone the screen will turn white or have loads of colours going across it. It still rings and i can answer calls but with the screen as it is i never no whos calling. Ive researched on the internet what it could be and dont no if to buy a new lcd screen or a flex cable ribbon, i would fix it in myself im just not sure which one to purchase. Any help would be greatly appreciated! Many Thanks :) x