Run through the contents of 'More like this' to the immediate right of your initial post. There are several people with this problem and one or two, especially in the link post below, Kaos2K, from Spain, who has what seems a realistic explanation
Thanks seventy one. I may try that fix but my problem does not present in quite the same way. Based on the photos and comments on that thread, their dimming/darkening/ issue seems to occur especially on the left side of the screen. My computer screen does a sporadic all over flicker with an occasional all over dim as if I had turned down the display brightness. So far the brightness always returns on its own after a time.
Harley, I can't say this has been resolved but I have a couple of updates. The flickering on my iMac became so frequent and annoying that I was getting ready to drive over two hours to the closest genius bar for help. Then the flickering almost completely stopped. My leading theory is that summer got so hot that I have had the air conditioner running constantly and that the flickering was somehow caused by the increasing heat and/or humidity in my home before I gave in to the air conditioner. Of course, that could be a coincidence. Maybe something resolved itself - hah! Maybe the flickering will return with a vengeance sometime. Right now, flickering is not happening.
On another forum, I was given this advice:
I had the same problem on my late 2009 27".
It is either the display or the graphics card.
If you can connect an external monitor to see if that works fine - if so, its the display -> expensive on my 27"
If the external monitor shows the same problem its the graphics chip - means replacing the motherboard.
I did not try this experiment because I don't have a convenient external monitor and the flickering only happened when it wanted to. I couldn't make it flicker. I think it is a good suggestion though.
Hi i experienced the exact same issues with my 21" iMac from late 2009 a few months ago.. Started with flickering and the brightness went down.. i didn't care too much then. But a few week ago it became really annoying!
And a few days ago my screen went all black! The computer works fine because i have a second monitor that i now use as a primary! And everything runs good.. But the iMac screen is now black all the time..
when i start up the computer it can come on for a few seconds but immediately goes back to black!
Have you experienced any of this? Or did it just stop? what should i do?
very similar issues with sporadic screen dimming and brightening with my 27" iMac Mid 2011. running 10.7.5; processor: 2.7 GHz Intel Core i5; 20GB memory.
no apparent rhyme or reason. flickers now and then. started about a month ago. i've read macfromrome's latest tread regarding testing with an additional external monitor and would like to find another way to troubleshoot as again, no rhyme or reason when it does it and this machine is my work horse.
macfromrome's theory about heat and using an air conditioner may have some validity. i work in a basement with a steady temperature of 19 degrees celsius. i sometimes reach behind the iMac for stuff and it often feels very warm. so wondering if it's a overheating issue.