Flickering squares colored forming strips on the screen

I have an iMac 27 mid-2010 "with a videocard ATI Radeon HD 5750 El Capitan running.  I have recently begun to see small squares colored screen (pink, orange, red, green mainly) that start as low spread across the screen and little by little become denser with time.  Places at their peak form a checkerboard pattern forming bands across the screen, but generally not in the active window.  Sometimes, I have a few strips of squares flickering and some fixed.  The individual squares are really small - only a few (2-3) mm in size.  I also occasionally get a purple line or two parallel lines of purple on the top of the screen (but not in the active window).  The points seem to appear when the internal hard disk is done a lot of read/write (but I couldn't be sure that it is a part of the problem). I have attached a screenshot that shows a few random points - he gets much worse than this.  Maximize the active window frequently, but not always, makes them disappear.  If they appear in the active window (which happens rarely), the computer stops responding.  I also seem to be getting some flickering when scrolling windows or opening menus, or an area of the screen not refreshing immediately, which I've not seen before.  The computer in all other respects seems to operate normally.

The purple strips at the top of the screen do not appear in this screenshot, but points are hard to see.

First of all, the problem occurred after the upgrade to El Capitan.  I made a second installation of El Capitan in case it was caused by the upgrade nothing helps.  It is currently running 10.11.2.  The computer also had a replacement of that HARD drive internal installed a few months ago, but the points started to appear some time after that has been done.

Someone has encountered this, or know a solution?  I'm starting to think that the trip of the new year for the doctor to Mac may be needed.

Thank you

I'm starting to think that the trip of the new year for the doctor to Mac may be needed.

Yes. Looks like it's maybe a faulty graphics card.

Tags: Mac

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