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iMac screen turns funny colors and computer itself freezes.

The iMac switches on and boots without any problems, then after about 15-20 minutes the computer screen goes lots of funny colors and anything the computer is doing stops working.

for instance, if the computer is playing music the song will get stuck on the second it switched to the purple green screen.

I thought it may be a heating problem because once the computer has been switched off after the problem it does not start again properly.

I cleaned out all the dust stuck in the fans and the rest of the computer to no luck.

anyone have any ideas?

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I wish I could find somebody to do this for me because I am sure I will &&^& it up on my own


I've had this issue recently with my 2008 24" iMac (8,1). Very annoying. Tried zapping PRAM and VNRAM reset but to no avail.

I agree that I think it's the GPU. In my model there is an NVidia GeForce 8800 which apparently has become known for breaking after 2 or 3 years because of heat. I'm going to send mine to be looked at, but the diagnosis alone is £50 then a repair could be at least £400 : /


Same problem with the tiny squares, but it freezes my entire iMac, except I can move my mouse, but left-click / right-click doesn’t do anything. Also, when I start up, the white screen has purple/green vertical bars along the screen


I got the color squares to not appear by going back and running a very early version of OS X, like 10.8, then restarting with latest version that had the colored squares problem previously.

Seems to have cleared video card sectors having the issue.

It's like magic for me.


@villielink - Colored squares is a symptom of overheated VRAM chips on the GPU board!

Basically, the VRAM is getting corrupted. You want to check your fans and you likely need to refresh the thermal paste & thermal pads on the GPU & CPU's.

If you don't fix this soon you'll likely kill the VRAM chips.


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Try zapping the PRAM and a NVRAM reset

Hold (command-Option-P-R) at restart through three chimes. Followed by a (command-O-F) restart hold the keys down until text appears on the screen type "reset-nvram" (no quotes) hit return type "reset-all" (no quotes) hit return see what happens.

If neither of those work then it could be the video card is going, OR that the thermal paste needs replacement. causing video problems and freezing due to overheating.

Good Luck,


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+ Ralph


THANK YOU! It worked and I owe you a beer or something!


Hi @originalmachead - I just tried your suggestion and so far, my problem has not yet re-occurred. I hope it holds up.

That said, I would like to try replacing the thermal paste. I could not find a guide for this specific model from this devices main link on ifixit:

iMac Intel 27" EMC 2429

Can you (or anyone) please send me a link to a guide on how to do this?

Thanks in advance.


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I've had this issue with a 27" mid 2010 iMac with a ATI Radeon HD5750 1024MB video card. I was seeing lots of small randomly colored squares appear all over the screen. Moving windows around would clear them out but the squares would reappear randomly over the screen just as quickly.

After seeing a number of posts suggesting this was a GPU memory issue and that reflowing the GPU circuit board might solve it I thought I may as well give it a go. In the worst case I would be up for a new video card, which was the next cheapest option anyway. In the best case it would cost me a $7 tube of heatsink paste.

I followed these instructions to remove the GPU, stripped off the heatsink, the foam bits, and a tiny white sticker and then baked it in our kitchen oven. Take photos as you go so you can remember where to put things back and where to re-apply heatsink paste.

Based on comments from other posts I followed these steps:

  1. scrape off any excess heatsink grease from the chips on the GPU and the heatsink itself,
  2. pre-heated the oven to 150c,
  3. very gently put three small wood screws into the mounting holes in the circuit board to act as spacers,
  4. placed some baking paper on an oven tray,
  5. sit the circuit board on the tray with the screws acting as legs or spacers to keep the board about 10mm off the tray,
  6. turn the oven up to 180c and give it a minute to adjust,
  7. turn the oven up to 200c and leave it for 10 minutes,
  8. turn the oven off, leave it to cool slowly with the door closed (I left it overnight),
  9. apply fresh heatsink grease to all the chips,
  10. reassemble the heatsink,
  11. reverse the steps described in the guide to put everything back together.

The hardest bit I found was reconnecting the the GPU temperature sensor. It's a tiny two-wire connector that gets plugged into the underside of the mother board and is in a fiddle place to get to. Right angle tweezers would be handy but I got but with a pair of mini needle nosed pliers.

So far it seems to be working fine. Normally the problem was showing up immediately after logging in to the iMac, regardless of if it had been powered down or left running for a while, but after an hour or so of running a bunch of apps I haven't seen it yet. Fingers crossed it lasts.

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