I have a Sanyo 46 inch flat screen hdtv purchased

I have a Sanyo 46 inch flat screen hdtv purchased from walmart aug. 31rst, 2008. Sure enough, 13 months later my tv is having picture problems and getting worse. It started having portions of the screen darken, initially towards the left hand side that would dissapear after the tv warmed up and would be fine the rest of the day. There were horizontal lines with the top of line ok and the bottom of line area darker. Now it's getting worse and the lines no longer disappear after warming up. They disappear and reappear at random troughout the day. Upto two thirds of screen may appear dark at times. What is wrong and what will it cost to have it repaired?