According to the Apple support team I contacted, it's an LCD malfunction and the only solution is to replace it. What they didn't say was why this problem is occurring after two years of using the iMac on so many computers. So, as Apple hasn't yet assumed that it's their fault, what I did, given the lack of concrete answers from Apple, was to take my iMac to a technician and ask him to assess the problem to see if it's a construction problem or just a random fault. The explanation I was given was as follows: In terms of circuitry, everything seems to be working properly. The problem, apparently, lies in a cable that also powers the LCD, which is located on one of the tops of the screen, and which, to be replaced, requires detaching the screen glass from the LCD itself, which is a very sensitive operation and almost impossible to carry out without damaging the LCD. So the only option is to replace the entire LCD (LCD+main boards+screen glass). Also, according to the technician's explanation, this cable (of...