DIAGONAL LINES LG PLASMA

Question

Hi there, we have a 42" LG Plasma approximately 4 years old that has developed multiple diagonal lines in the picture - probably started about 4 months ago. They seem to vary in thickness while watching - thus thinner lines making it easier to see picture. It doesn't seem to get any worse just varies in the thickness during viewing time. They never disappear. Have tried all different inputs and have the same problem watching DVDs (different input). Had a service company look at it and they suggest it needs a new "main board" @ $900 Australian? Any suggestions? Thanks, Shelley.