27" iMac (late 2012) display issue

Hi there,

About a week ago, the screen on my 27" Late 2012 iMac, started flickering and distorting (screenshot here: http://i.imgur.com/eIb8uZA.png)

I reset the SMC and NVRAM, no change. Ran short and extended hardware diagnostics tools, but this unfortunately didn't find anything. The strange thing is, if I mirror the screen to my TV via airplay on the apple TV, the picture is great: no flicker or distortion. I thought it might be the video card, but not being an apple technician, things soon stopped making sense.

Physically the iMac is in excellent condition, free of dust, and has lots of dry air moving around it. It is out of the standard 1 year warranty from Apple, and it seems like I didn't purchase the extended warranty (can't find the invoice).

Action before this occurred: I plugged in my iphone 5. Before that, it was left alone for some hours.

It's been a fantastic machine up until this point, so does anyone know what might be the problem and how I might get it fixed? I'd rather not take it in to someone who will charge me for a new graphics card or else.

Model details as follows:

Model Name: iMac

Model Identifier: iMac13,2

Processor Name: Intel Core i7

Processor Speed: 3.4 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 8 MB

Memory: 8 GB

Boot ROM Version: IM131.010A.B05

SMC Version (system): 2.11f14

http://i.imgur.com/eIb8uZA.png

Graphics:

Chipset Model: NVIDIA GeForce GTX 680MX

Type: GPU

Bus: PCIe

PCIe Lane Width: x16

VRAM (Total): 2048 MB

Vendor: NVIDIA (0x10de)

Device ID: 0x11a3

Revision ID: 0x00a2

ROM Revision: 3707

OS:

System Version: OS X 10.10.2 (14C109)

Kernel Version: Darwin 14.1.0