Mid-2011 27" iMac constantly freezing/ restarting

Question

Hey there, so I'm at the end of my tether here because I've barely been able to do anything on my iMac as of late because I've been experiencing severe freezing issues.
I'll start from the beginning:
I've had the iMac since mid-2011. It's a 27" iMac with a 3.4 GHz Intel Core i7 processor, 8 GB 1333 MHz DDR3 memory, and AMD Radeon HD 6970M 2048 MB graphics. It's served me so well over the years, but 2015 has been the start of something hellish.
I first noticed it a month or so having upgraded to Warlords of Dreanor in World of Warcraft. This is the game I primarily play, and have played for years. The graphics have consistently been played on 'ultra' (the highest setting) and have been as smooth as butter since I can remember. But as I say, a month or so into this new expansion I notice that everything's loading up a lot slower than previously. Previously when I minimised the game into windowed mode, it would work in a flash, now it would be sluggish it resets the game's UI overtime I did so. I lowered the graphics thinking perhaps this was the problem, but it wasn't. Then the crashing started happening.
I would be playing, and suddenly my iMac would crash. The screen would look like it glitches out with an array of pixelated colours, then turn one of a numerous amount of colours with a corresponding colour repeating as vertical stripes. It would remain this way until I had to hard reset it with the power switch. This persisted even when not gaming now. I would be watching a youtube video, or refresh tumblr, and the same thing would happen.
I took it upon myself to see what could be up and I found this iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program - Apple Support
Thinking this was the problem, I booked my iMac to be seen with a genius.
Now with my iMac infant of a genius, he ran whatever stand tests they do with iMacs on my type, and said the graphics card wasn't showing up with any particular problems. Nor anything else. He took it in though so they could run further testing. A week or so later I am told there's nothing wrong with the graphics card but a fan doesn't seem to be working and they can replace that at a cost & labour charge. My receipt lists the part as "FAN, HARD DRIVE", it was only cheap (£13).
I get my iMac home, thinking it's all good to go! and... it's bad. It's really, really bad guys.
Now for whatever reason I do not know, my iMac starts glitching out on me for what seems like nothing!
I startup the iMac and it will always spend a minute or two on the white screen with the loading apple. Once I'm logged in, it may only be a minute or so until I'm hit with the screen turning one of a numerous amount of colours with a corresponding colour repeating as vertical stripes. If I'm logged in for long enough to have opened iTunes and got some tunes playing - the music will stutter in the background on the crash and within a minute or so the iMac will restart itself.
Here's the thing though, this crashing is so sporadic. I've kept a log in my iOS notes of when it happens, what I had open when it did, and what I was doing, and there's nothing of any correlation! In fact, the longest time period I didn't experience a crash was during a few hours long session of Diablo 3 (graphics high).
Unfortunately it's not a technical log of any sort, all my logs are akin to:
12/02/15 21:16 watching Netflix full screen through chrome, Netflix was paused, when going to unpause screen goes a cardboard orange colour with faint grey vertical lines. After less than a minute iMac restarts itself.
Taking photos whenever I could of said crash, like so:
I was at the point this evening where I tried on 4 consecutive restarts to get a game of Diablo 3 going and couldn't get more than 1 minute into playing before this crash happens.
What on Earth could be the issue? Because the Genius guys certainly didn't find any problems after their week long 'intensive' checkup...
Thanks all!

Answer 1

I've got the exact same machine and I also had a graphics card issue.
When the Mac starts-up, the graphics card on the Intel processor does
all the initial work. If you get to the stage when you see your user accounts
you know that the graphics processing has been delegated to the on-board
dedicated graphics card.
If you do get to this stage and it holds steady, it's probably not the graphics
card that is causing the problem. When mine crashed, it also took out my
USB ports and I also had a corrupt hard drive! Don't judge a underlying
issue by it's apparent symptoms.
My problems started in September 2014. If you are in the UK, judging by the
£13, you have 6 years for your machine to function reasonably properly and
for the reseller directly (Apple indirectly) no fix any issues free of charge.
Apple does advertise it's 6 year liability to comply with the Sale Of Goods Act
which is unprecedented in the commercial world so five stars their! But your
legal contract is with the reseller. You are also entitled to any costs you
incurred like delivery of the machine.
I wrote to my reseller and they agreed without question. After signing some
papers to agree with the Apple part replacement program or something it's
been running as new.
If you haven't done so already, contact the re-seller and explain the problems
and show them the evidence you have. If that fails then you should be able to
take the machine to a Apple specialist for it to be diagnosed with all costs
covered by the Apple programme.