Hello, I'm having issues with my iMac (27', late 2009), and after some troubleshooting still can't figure it out so I thought I'd ask here.

The first time I noticed a problem was when I booted up a couple of days ago. The screen had random pixels transplanted all over the place, and if I attempt to perform simple tasks such as moving windows or simply clicking on icons, the system freezes up -- when that happens I can still move the mouse pointer around but that's all I can really do, and have to hard shut down.

If you’re comfortable with it, open up your PC and remove the RAM from your computer. You can unlock the locks on the memory sticks by pressing down on the little plastic tabs at each end of the module. Next, insert and test one memory stick at a time. To cover all your bases, you should test that memory stick in multiple modules on the motherboard.

I then tried using the CD with the Apple Hardware Test (i.e. together with holding 'D' key down while starting up), and it revealed the following error code --

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4MEM/9/40000000: 0xb78df418

Upon researching this error code, it appeared that it was pointing to my RAM as a potential issue. I thus proceeded with the following steps --

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1) removed and re-inserted the 4 RAM cards (no difference in symptoms)

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2) removed 1 RAM card one-by-one and tried starting up (to see if any 1 of the RAM cards was faulty, but no difference in symptoms)

3) tried starting p with just 1 or 2 RAM cards in certain RAM slot configurations

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#3 above is where it got interesting. I found that if either 1 or both of the back pair of RAM slots were used, and the front pair left empty, I would get 1 beep every 5 seconds upon starting up. If I just use 1 back and 1 front slot, the rig starts up fine, but still with the pixels. According to this, that's telling me the back pair of RAM slots is bad? The thing that confuses me is that when I only use the front pair of slots, I still get the pixels. So do I have a pair of bad RAM slots, and 4 bad RAM cards too? Is it possible for bad RAM slots to cause all RAM cards (even those in good RAM slots) to fail?

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iMac, OS X Mavericks (10.9.3)

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