Pink and green square dots all over my iMac Screen

Hello, my 27 inch late 2013 iMac is full of random dancing pink and green squares all over the screen. I just updated to El Capitan and it did not fix it. When I start the computer, it does not show until I open a video, quicktime, vimeo, youtube, finalcut pro x, anything. But it does not bother when I use adobe illustrator or photoshop.

It is also showing this very strange checkers. It happened when I open a second tab in chrome

If I share my screen on apple TV, I will see the problem on the TV, exactly the same dancing squares. But it sometimes, the problem disapears. It seems that the video card is forced to work right for a little time

If I render a video in FCPX, I get to see the square dots on screen, but then I play that video on another device and it runs perfect without chickenpox.

I think it is the video card, and at the apple store technical center, they gave me a quote to fix it that costs almost two times what I payed for the iMac. Does someone else haves the same problem?

This is my system, please help

iMac (27-inch, Late 2013)

3.4 GHz Intel Core i5

8 GB 1600 MHz DDR3

NVIDIA GeForce GTX 775M 2048 MB

OS X El Capitan, 10.11

Thank you