My 27" iMac suddenly died reading a web page as if there was a power cut. On trying to reboot, one is faced with the typical grey screen and progress bar along with a number of quite broad green vertical lines. Progress stops, then blank white screen appears, then a reboot back to grey screen and green bars... And again. I've googled a lot but with no definitive answer... Logic board? Graphics card issue? Some help would be appreciated.