Q: Red and green square dots?

PhilP2 - that's the same system readout as the one from my OEM ATI Radeon HD 5750 thaet came with my mid-2010, i5 27" iMac...

I'd run for over a year without pixelations... until upgrading to Yosemite a couple of days ago.
Checked to see if my "energy saver" settings had been restored to their defauilts (NO).
Problem seemed to increase with mouse movement or typing...

System then froze: Had to hard restart - twice (interrupted the first restart with a second, when it stalled).

Now, no problems reoccurred and strangest of all - my display has the richest colors I've (never) seen since it was purchased - more vivid, sharper, deeper!

I might also add that prior to this reoccurrence, I'd been running a small fan across the display top to  more quicky dissipate the heat that it was emitting - and after reading several posts suggesting higher temps seemed to alleviate the issue for some. I thought perhaps returning to "factory cooling specs and conditions" might aid.

So far, it seems to be working... and I'm thoroughly enjoying my "new" monitor!
Does anyone have an inkling as to why it seems so MUCH better (other than to postulate that - as a light bulb about to burn out - it may be brighter and better prior to imminent failure)?

I had my video card replaced twice and my logic board on this 27" mid 2010 iMac and it got rid of the problem for a good year. Ever since I installed Yosemite...it came back and even worse it locks up my computer. When I did a hard reset it reported a "Graphics Error" to Apple but didn't give me a crash report.

This is depressing.

Here are some new members to our club: Imac display issues following Yosemite upgrade

Ok so I got thinking is this environmental? Last winter when temperatures reached freezing was when my iMac started to experience these problems and my computer is in a cold room next to a drafty window. I had the video card and logic board replaced and it's been fine until now when the weather started to get cold again. Could the heat of the iMac combined with the cold temp cause the thermal paste on the GPU to crack and flake off?

It's kind of similar to the ATI cards in Xbox 360 where people wrapped a towel around them to try and reflow the chip. Is it just that? I'm going to try to raise the temp in this room to see if that makes any difference to the frequency of my pixelation problems before I consider baking the video card.

You are right. The colder environment makes the symptoms worse.

But the problem is not the thermal paste. It is the cold solder joints on the graphics board.

I know because I first tried removing all of the thermal paste and installing new paste.

That did not change my symptoms at all.

Then I baked the board for 7 minutes at 390 degrees F

That fixed it.

You are wasting your time playing around with fan speeds and room temperatures.

Just bake the board and be done with it.

I'll order up those giant suction cups and follow the iFixit guide once things get worse. The fix is so hilarious and bizarre to me that I look forward to doing it.

6th day out from my "cook" and things are working great.

I would second the recommendation earlier not to disconnect the GPU connector from the back of the motherboard.  Very tight and hard to get to connection.  I left it intact.  Not an issue.

I was able to pull the screen off without the suction cups, just slowly and carefully with my fingernails.  Again, its just on with magnets.

Follow the guidelines online and make sure the card is up on something like the wadded up tinfoil balls like seen online.

Go very very slowly as you take things apart.  Lots of little things to break.

For the rest of you, I tried all the tricks on here, but in the end, I have to agree that this is a HARDWARE issue and no software fixes are really going to resolve anything long term.

Hi everyone, I'm back again after many happy months of working graphics, but as temperatures dropped everything came back.

Back in april I removed the 5750, applied new paste, and made some attempt at reflow with a heat gun. I was too afraid to put the poor thing in the toaster oven.

My iMac was good as new for nearly six months!

As temps fell the squares started to return, but without crashing. Then I installed Yosemite. Worked well for a day, then one more cold night and I would see the worst I've ever seen in the many years I experienced this problem! The computer would hard crash within a minute or two of starting up, over and over.

Now that I have your attention, just a few hours after that photo, my iMac is back to good as new! As it was now rendered completely useless, I had more confidence and determination than ever to stick the graphics card in the toaster oven. Plus practice means I had everything out in less than an hour.

Five minutes at 450. Good for pizza. Good for Radeon.

We'll see if I get another six months, but it sure beats spending $350 on a new card that probably will still have the same issue! Apple, feel free to donate a new 5K iMac four my years of trouble with this!

I should say, months or years spent wondering if this was a software, kernel, driver, or even card connector issue. It is definitely a cold solder joint issue, which gets worse with time and low temperatures, and reflow is the only answer!

Hi all - I'm having the same issue with mine after upgrading to Yosemite (it presented itself a little bit in Mavericks as well), so I have a lot of info to weed through here. Just letting everyone know there's one more mid-2010 i7 iMac with the ATI Radeon HD 5750 1024 MB graphics card having issues. Will continue to monitor the thread for progress, and hopefully I won't have to bake my card!

Yosemite might give us the last glimmer of hope that Apple might do something about this. Not only is it pushing many more imacs over the edge, it appears it is able to recognize the graphics issue and reports it to apple. Of course, these machines are four years old, so they don't have to care. (It is clearly AMD's manufacturing issue anyway)

If there is interest I would be happy to write up the full process and post on my web site. I have photos of most of it.

If you have plunger, you can use that to remove the screen.  I have removed my screen several times using only a plunger but I dare say I could use my fingernails as suggested by KirkB. 

If you have the time and don't mind, a writeup would be great. I've ignored this issue, which has honestly been minimal, until the upgrade to Yosemite so I'm on the start of my journey here. To see what happened, the process you went through, and what worked/didn't work is certainly of interest to everyone having the problem. Thanks!

Josh, that would be very much appreciated - thanks!

Warming up the work area did help. Usually every morning I find my computer with the flickering pixels and I have to restart. I'm sure this is only temporary but it does prove to me it is cold solder joints. I will be definitely baking my card in the near future as well as replacing the hard drive with a SSD to give this computer new life.

A thorough guide with pictures on how to bake would be much appreciated. I know when I do it I will film the process for YouTube as I spent almost $600 at the Apple Store last year replacing the logic board and video card because the Geniuses couldn't figure it out. I'm thankful to everyone in this thread.

I took my mid 2011 iMac (21.5) to the apple store and it failed a heat sensor test (the one on the processor). They had mentioned off handedly that it sucked in dust, so I asked them to attempt to open it and give it a thorough cleaning if it needed it. They did end up keeping it for a week, opened it and cleaned out "a lot" of dust and then ran it through diagnostic testing for a week and couldn't get the heat sensor to fail again and could not get the dots to reappear.

So, it may indeed be a heat issue caused by several different kinds of issues. Mine was simply a build up of dust.

There was no charge for having them clean my machine by the way.

I have a few tips after sucessfully baking my graphics card...

1. This might be obvious to most but be sure to separate the graphics card from the heat sink before baking.

2. Have some fresh thermal paste on hand before starting the job. The old thermal paste will be dried out.

I removed all of the old dried out paste using a putty knife and put new thermal paste back on the heat sink.

(after baking the graphics card).

3. Baking temperature and time.

I did some research on this and in general, solder starts to melt at 350 degrees F (Fahrenheit)

And solder becomes fluid at 450 degrees F

So I picked 390 degrees as the baking temperature for a kind of middle temp. I am guessing that is high enough to re-flow the solder without worrying about the solder dripping off of the board.

I just picked 7 minutes for the time because someone else suggested it.

4. When placing the graphics board in the pre-heated oven, make sure it is sitting on a baking pan (not just sitting on an open grill.

Use some sort of standoff to keep the board from touching the pan.

Some people use wadded up aluminum foil balls because the foil will not adhere to solder. I used four screws as standoffs because there are four handy holes in the board to insert the screws.

The idea is that you don't want the solder joints on the board to contact the metal baking pan.

5. After baking, turn off the oven and open the oven door to let the board cool back down.

I waited a couple of minutes before removing the board from the oven because you don't want to move it while the solder is still in a molten state.

That's all I can think of for now.

Unfortunately, I did not take pictures, so I will leave that up to others to do.

I know that sounds like a crazy thing to do but it actually works.

Rich