Screen Fault in on a Plasma?

Hi,
I have an LG 42 inch plasma TV. Its about 4 years old but recently the picture has started to play up. Difficult to explain but it flashes white but you can still make out the picture and it goes into a kind of negative picture. Its intermitant and seems to do it more when its just been turned on and settles down after its been on for while. Can anyone help, is it dead ?