It has happened 2 or 3 times on random occasions months appart. but say after having my imac on for long periods of time for a day or two downloading files or using final cut etc.etc. anyway it sounds like the harddrive whiirs to off, like when u put it to sleep get the same sort of noise, or it may be a internal fan not sure? anyway then the screen will flicker on and off violently not totaly black but off with the backlight still on and eventually it will stop with nothing on the screen and no noise coming from hd the screen is blank but im pretty sure the backlight is still on. after a few hours rest it appears to be buisness as usual so i think it may be a overheating issue or maybe harddrive just being over used? anyone experince simular issues or no what is going on and is it worth a visit luggin this beast into a apple store lol TY!