Graphic noise in OSX
I'm sorry to break the bad news, but your GPU has failed. This is a known issue for your machine. Your model was subject to a replacement program for the logicboard due to a defective GPU, though it no longer is active. Apple may or may not still replace your logicboard. It is ultimately at the discretion of the store manager. If you don't qualify, your other option would be a flat rate repair, for ~$300, which Apple will repair anything wrong with the machine for this fee. Just googling around, other options exist that may prolong the life of your machine. 

Ultimately though, this is a hardware issue for which no online troubleshooting exists. Best of luck.