Some models of 27″ iMacs sold between May 2011 to October 2012 may exhibit video problems including: white or blue vertical lines, distorted images, or the display turns intermittently black. These iMacs have AMD Radeon HD 6970M video cards, which came with both the 3.1GHz and 3.4GHz Intel i-series chips.
Apple has announced a program to replace these faulty graphics cards, even if the iMac is out of warranty. Apple is also reimbursing owners who previously paid out-of-pocket to have this repaired. For more information, visit http://support.apple.com/kb/TS5167