iMac 27'' mid-2011 stuck on startup (grey screen)

Question

Okay, so I'm having the dreaded grey screen on my iMac - precisely :

1. i'm having it doing a normal startup
2. i'm having it trying to boot from recovery partition
3. i'm having it trying to boot for internet recovery
4. i'm having it trying to boot in safe mode
5. verbose mode is no help because it ends with a grey screen before I've time to read anything
6. i have a windows partition which also fails to boot, except in safe mode - from where I can access all my HD partitions without any error by the way
7. fsck -fy in single user mode reveal no volume error
8. an extended Apple Hardware Test reveal no hardware error whatsoever
9. replacing RAM modules didn't change anything

So I'm stuck : what can possibly cause the problem if it is neither the OS (remember I'm having trouble with both MacOSX and Windows), and nor the hardware (at least if AHT is indeed reliable) ? I thought a driver might be involved, but I haven't upgraded or changed any driver on my machine since ages ago, and it worked perfectly fine until last sunday.

If anyone has an idea, please help, because I can't even point out support with a possible cause for failure here.

Thanks

Answer 1

You probably need your video card replaced. See this Apple article regarding a known video card issue:

https://web.archive.org/web/20130820015305/http://support.apple.com/kb/TS5167

From the article:

Apple has determined that some AMD Radeon HD 6970M video cards used in 27-inch iMac computers with 3.1GHz quad-core Intel Core i5 or 3.4GHz quad-core Intel Core i7 processors may fail, causing the computer’s display to appear distorted, white or blue with vertical lines, or to turn black. iMac computers with affected video cards were sold between May 2011 and October 2012.