have you recently moved the computer around at all? when i flipped my imac over to put in more ram, the upper right hand corner (not the rest oddly enough) flickered for about a week. the bottom right hand corner is now a little bit lower than my bottom left hand corner, but it's not really noticible. you may have jiggled something in there and it could be your problem. make sure that you have the latest firmware update as well.