iMac 27' (mid 2011) fix for AMD Radeon HD 6970M no video problem - YouTube
New HD 6970M 2GB Graphics Card Replacement for iMac Mid-2011 27" All-in-One Desktop PC Core i7 3.4 MD063LL/A MD063, AMD Radeon HD6970M HD6970 GPU Upgrade MXM Video Board Repair Parts : Amazon.ca:
Genuine Graphics Video Card Replacement AMD Radeon HD 6970M GDDR5 1GB 109-C29657-10 for Apple iMac 2011 27" MC814LL/A MD063LL/A A1312 Desktop PC Computer, MXM VGA Board Repair Parts Full Tested : Electronics - Amazon.com
Apple Initiates Video Card Replacement Program for Radeon HD 6970M Equipped Mid-2011 27” iMacs