I'm after some advice please, the screen on my old iMac (c. 2007) just pixelated. I can access files and the software all seems to work as normal, but I am struggling to see past the pixelation. It covers the whole screen in non-uniform broken horizontal lines (some black and some white and of varying widths). This problem happened about three months ago and fixed itself after a few re-boots, but that's not working on this occasion. Any ideas please?