I've tried searching for a reason as to why my iMac (purchased new in October 2011) has suddenly started getting this issue. It first occurred Sunday night, and has happened just under 10 times since. The first few times the screen would flicker as if it had a bad TV reception, followed by the entire screen going white (the boot up screen minus the apple logo). Now however, it's not giving any flicker, it will just go white without warning. Most times, but not all if I remember correctly, had YouTube running either at the front, or running in a tab whilst on a different one. I saw somewhere online that recommended holding Command, Option, P and R before the white screen during boot up, to reset the PRAM, which I did last night. It appeared to be fixed, but then I had the screen turn white on me this morning. Details about my 27" iMac (I can provide more if required) : OS X - Version 10.7.3 Processor - 3.1GHz Intel Core i5 Memory - 4GB 1333 MHz DDR3 I've looked online, through various Google results, but unfortunately most people seem to get a black screen which indicates the backlight has gone. It seems like I'm getting the opposite of that. I even tried signing into Apple Support Communities but the sign-in system glitch prevents me from doing so. Top work, Apple. Any help on this matter would be greatly appreciated. Thank you.