iMac screen flickers, freezes, crashes

For several months I have struggled to resolve an issue with my early 2009 24-inch iMac 3.06 GHz Intel core 2 Duo. I have visited two Mac repair places, one of which took the machine to the Mac bar and had the power supply and graphics card replaced, but nothing has solved the problem. Neither repair place could fault the machine in testing.

Symptoms include a flickering screen and screen tearing (small portions of one part of the screen appear on other parts of the screen), strange pixelation often in green and purple, followed shortly by the computer freezing. Sometimes if I leave it for five or ten minutes the mouse comes back to life but nothing else and then I'll often get kernel panic. Heavier graphics use (photo editing or image-heavy browsing) seems to make the problem present sooner - within half an hour.

Sometimes, if I have music playing when it freezes, I can hit the volume button and about ten minutes later the volume will change accordingly. This hints at (severely delayed) hard drive activity in the background.

The latest repair place (Mac & PC Doctor) drafted the following report:

"• Ran Memory Test over night - on all Installed RAM to confirm that the Linear PRN and stuck address is returning correct information- All ram memory test passed..... No fault found.
AST (Apple Service Test) Ran: Tested Logic Board• SD Board• Bluetooth • AirPort Display • Camera •Apple USB Input Device • Optical Drives • Ethernet •Hard Drive • Power Supply (desktop Macs only) • Fans, GPU (Video Controller)• Sensors •Processors & Memory : Pass - All components Present
Tested via customers OS - No fault found
Having troubles faulting the unit.
All HW passes all of our tests which indicates a intermittent SW fault."

They suggested the problem could be an intermittent power supply in my flat. I have moved the computer to a different room but the problem persists. I have considered buying a UPS but I have serious doubts that power supply is the problem given it has only presented in the last four or five months and I've been in the flat for much longer.

I have reinstalled OSX Lion. I have been running a piece of developer software called Quartz Debug, with which I have been disabling Quartz Extreme and 2D acceleration, which while diminishing graphic performance seems to allow the computer to run for longer without freezing and/or crashing. But in the end it always freezes.

The machine is out of warranty. It is basically unusable in its current state. I really am at a loss and can't think what else to do other than to throw it in the bin and buy something else. Any assistance would be greatly appreciated.

Most odd, sounds like a video card problem but you've had it replaced already.

If the fault is intermittent it can pass all of Apples tests. The report you quoted references Apple Service Test, which doesn't exist. There is an Apple Service Toolkit, from which MRI can be run. It's a quick test, takes about 3 minutes and logs the result with Apples GSX against your Mac's S/N. The best test for probing an intermittent fault would be ASD, Apple Service Diagnostic. On newer Mac's it runs from an externally booting USB HD, and typically takes an hour to run. It can be left looping for long periods. Even this thorough test won't find a problem unless the failure occurs while that section of your hardware is being tested. How often does the fault occur?

As you've paid for a repair including replacing the PSU and video card, and the fault wasn't resolved, I'd take it back to the first repairer as they fitted and charged you for parts that weren't required.

Hi Steve, and thanks for your reply.

The fault occurs almost every time I use the computer. That is, I might get lucky and be able to use it for four or five hours if I use Quartz Debug to disenable Quartz Extreme. But a lot of the time it occurs within 10 or 15 minutes, especially if Quartz Extreme is enabled. Having said that it can often run for days at a time if it sits there without being used. I often leave it on to allow background operations to keep going like backing up to an online server and such.

I've been on the phone to the repair shop again today and they reiterated the power source theory. I tried a new power board with a built-in surge protector, a different power lead and a different power point in a different room in my flat, but the problem persists. I'm not convinced it is a power surge issue since none of my other appliances or laptops have faulted and while I've been living here for 18 months the problem only began in about March.

Do you have any specific instructions on how to run the ASD or the Apple Service Toolkit?

Thanks again.

ASD and the Service Toolkit MRI are Apple proprietary, for use by Apple or AASP's. There are multiple versions of ASD, each one covering a specific range of Macs. Apple Service Toolkit is again used by Apple and AASP's, net booting from OS X Server and linked by an account to Apple. It should be, if possible, run at least once on every repair and used with a customer as a quick diagnostic, and after a repair to show all is well.