iMac 21" Display square artefacts problem

Hello everyone!

I've already seen that there are several people having same problems (or almost the same), but i assume not quite the same.

Problem is - yesterday while watching just a film suddenly I saw strange colored squares all over my 21" iMacs (late 2011) internal display. Mac has been working for 1.5 years only (quite a shame...) Guarantee has already expired.

I saw that the squares refer to particular window - when moving a window all the squares existing on this window move with it.

Screenshot: http://rghost.net/49559599.view

There are no squares on dashboard background, in the recovery mode and on startup until login window appears

Dashboard: http://rghost.net/49559721.view

Recovery: http://rghost.net/49560165.view

If I set a white solid background of my desktop - there're no squares as well.

White one: http://rghost.net/49559817.view

Apple Hardware Diag didn't show any troubles.

Complete erase all and clean OSX (10.8.5) re-install didn't change anything.

Fan control showing both (left and right) fans are working, can't be a heat issue.

Color Depth change didn't solve the problem.

Reseting PRAM/NVRAM didn't solve the problem.

My next step will be checking the output on an external display. Opportunity will be in a week only 😟

The only thing i can't explain - why there're no squares in recovery-mode, as windows and background look the same as in normal mode.

Anyone any suggestions? GPU or rather display panel? Would really appreciate any help.

P.S. If it's a display panel issue - I think I won't pay 600-700 EUR for replacing it.