If you don't have time to read my story (which also describes some interesting technical approaches) just scroll down this thread until a "100% WORKING SOLUTION" text
Discrete AMD GPU of my 2011 MacBook Pro 8,2 has finally failed because of the reasons mentioned here (...
Follow the steps described in this guide:
Installing the attached kext to prevent the GPU from waking up from sleep (optional)
WARNING: This kext should only be used with the GRUB solution in place, otherwise...