Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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Is my graphics card failing?

I have a 24" aluminum iMac 8,1 - 2.8 GHz, 4 Mb RAM, Model A1225, EMC 2211,

Very recently my Mac started having major problems related to freezing and screen fragmentation and flickering. Windows become mangled and off screen. Even menu text looks a little weird. Fans/temps are fine.

Trouble first seen during some app updating and deleting, yesterday, but I'm not sure if this is a coincidence.

Reboots seemed to help, but it got worse, now starts before boot up completes. The computer is basically unusable at this point.

Graphics card is NVidia GeForce 8800 GT. Hard drive was replaced a couple years ago to a 2 TB SSHD. The HD looks fine by Onyx and Disk Utilities. Avast antivirus is clear. AHT only shows 4MOT/1/40000003/HDD-1374, which is my HDD fan speed.

I'll try safe boot tomorrow, but I wanted to see if anyone has seen this, is it an obvious graphics card failure? I'm running Yosemite, and was considering El Capitan, but I wasn't ready to pull the trigger yet.

Update (01/03/2016)

Thanks Dan. Yesterday, I did the safe boot. It shut down 3 times at about the half-way point, then finally completed. The graphics issue was still there.

I haven't been able to figure out how to post a screen shot. I'm not sure what/where the media manager is, and dragging or pasting is not working.

After a fourth safe reboot, the desktop appeared normal. I took the opportunity to do a limited time capsule backup on a spare drive. I adjusted the energy saver so that the hard drive wouldn't sleep, and the monitor would sleep after 1 min. I periodically woke up the monitor and the backup was proceeding normally for a few hours. When the backup appeared to have completed without incident, I switched the energy saver to the default settings. The next time I checked, the screen was completely screwed up. I have been snapping pics, so if I can figure out how to share I will.

As far as the fans and temps, I originally had the SMC fan control. Everything appeared fine. I switched to Macs fan control (I can't remember the exact name). It also showed temps to be ok and all fans working. I have taken the computer offline presently, so I could set up a laptop station. I may try the TG Pro as suggested, but I think the fans and temps are not the problem, although the issue may get worse as the system warms up.

From what I've seen so far, the problem developed quickly, although not instantly. It started a little glitchy, and within a couple days became almost constant. It did not appear to be affected by a safe boot, so possibly not related to software. It seems to cascade to a crash when something is done that requires higher graphics processing. The hard drive seems to be ok. I don't know how to test further to be sure what the problem is, but I'm considering plopping down $200 for another NVidia card and hoping that swapping it out will fix the issue. I think I remember that I installed the current card shortly after getting the computer in 2008. Do they have a life expectancy? The computer has been vacuumed fairly regularly to clean dust from the screens, although I can't vouch for inside recently. The issue occurs upon the first bootup in the morning, so I don't think temps are the ongoing cause.

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If your system runs OK for a while (when cold) then I suspect you have a heat type of issue.

It could be a simple fix like cleaning the dust out as well as refreshing the thermal paste on the CPU & GPU chips. It's also possible the heat pipe has lost its coolant so the chip is not able to shed its heat. And lastly, as you have noted the graphics board may in fact failing.

I would start things off here by doing some deeper diagnostics. I find AHT is not the best tool to diagnose heat issues. I prefer using this: TG Pro it offers a visual breakdown of all your fans and thermal sensors. Give it a try and post a screenshot within your question so we can see. If you get the full app you can enable logging so you can see the time point the screen gets distorted as well.

Update (01/05/2016)

I like the last image could be modern art :-)

Sadly, it does look like your logic board is failing.

Between the graphics and the panic error the north bridge chip looks like it's going. This might be repairable by reheating the chip (or replacing it) but you really need to correct gear to do it.

But given the systems age it maybe better getting a newer system (used or brand new). Apple has just placed this series on the EoL list so they won't fix it and they will nolonger supply parts for it. That's not to say you still can't get a replacement logic board, only you'll need to do some hunting on the Internet if you want to try to fix this system.

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