My old iMac, a mid 2011 27 inch with the HD6970M GPU, had its GPU crap out for the second time a couple of months ago.
The first time Apple replaced it for free under a special program because it was flaky. I suspect the problem is the iMac gets too hot for it, for reasons that will become clear later.
it is almost time for this beast to be put out to pasture, but i have been booting via a thunderbolt enclosure, so it is pretty snappy, and so I was prepared to wait for a new iMac.
Anyway, a couple of months ago the GPU crapped out again and i had the pink stripes of death onscreen. What the heck, I thought,I will take it in to Apple Store and see what they reckon, they might have GPU out the back and still replace it for nothing. The person that served me ran through the diagnostics and said, yep, its the GPU. It will cost you $940 plus labour to fix. When I declined to spend that on a computer probably worth $600 she said, well it is getting old, why not replace it with a new iMac? I sensed they were a bit unhappy when at this point I said if I was replacing the iMac it would not be with anything in Apple's current lineup, why pay a premium price for a broadwell processor? They had to mumble some useless platitude. I almost felt sorry for them.
But I didn't replace it, So I have been living with an ipad or borrowing master entropy's HP spectre or miss entropy's MBA while waiting for a new model iMac. That was stupid.
Easter I was stuck at home with the flu, and the rumours are that there might not be a new iMac for six months at least. So I looked on youtube at some solutions for a crapped out GPU. Apparently this is a common problem with the mid 2011 iMac with this GPU. The solution is to take the GPU out and bake it in the oven for ten minutes. There are plenty of youtube clips of how to take the image part and do this. Not for the faint hearted though, and there are some design decisions that seem less about layout and more about making it hard for a user to repair, which is annoying.
It took me about five hours, including a couple of trips to jaycar for some thermal paste, and then to get a damn T9 screwdriver kit for the screws, and a trip to bunnings to get some electrical solder after master entropy accidentally stripped two of the wires from the wireless module (a couple of lumps of solder seemed to do the trick).
Anyway, after baking the GPU, I put it all together again, ended up with a spare metal clip and about five screws that didn't seem to belong to anything, and tried a reboot. Bliss! I will be happy for her to work until a new iMac comes out now. So, $6 for the thermal paste, $30 for a special screwdriver kit, $5 for a new roll of solder, and five hours of stress vs $1000k plus to replace it with a new one. It even runs quieter now that I removed all the dust inside. A win for once.
As to why it works, I think it must be that the solder in this model GPU cracks, breaking a connection. Baking it at 200 degrees melts it all back together again, and so it starts working again.