I get lines through my screen of my iMac. Is my screen burning out?

I frequently have lines through my iMac screen, usually while on the Internet. Also, the screen will occasionally go dark with the exception of a small, bright, white square (usually 1.5" by 1.5"). I've done a clean install of my software, to no avail. Are these symptoms that my screen is burning out. If so, is it worth the cost of repairing/ replacing the screen vs. buying a new computer?