Random green and magenta pixels – El Capitan

I still have green and magenta pixels randomly appearing on my monitor of my iMac 27″ (2010) 2,8 GHz Intel Core i5.

Mac OS El Capitan 10.11.1

I had it also with Yosemite 10.10.5.

I had no problem what so ever with Yosemite 10.10.4

I’ve done every thing… P-Ram, SMC, Hardware Test (No problem, by the way)

Someone from Apple who can respond on this?

I work with Apple since the Macintosh IIx (approx. beginning of 1989)