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