Here's the steps to replace your GPU card: [guide|5214|iMac Intel 24" EMC 2111 GPU Assembly Replacement]. But, I wouldn't replace the GPU card yet.
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Given the age of your system I suspect your is quite dirty inside and needs a good cleaning and the thermal paste on the CPU & GPU needs to be replaced.
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Given the age of your system I suspect yours is quite dirty inside and needs a good cleaning and the thermal paste on the CPU & GPU needs to be replaced.
I would start there cleaning out the dust and redoing the thermal paste. It just maybe the issue is heat. To help monitor things I would also install this app: [http://www.tunabellysoftware.com/tgpro/|Temperature Gauge Pro] so you can see the before & after conditions.
Here's the steps to replace your GPU card: [guide|5214|iMac Intel 24" EMC 2111 GPU Assembly Replacement]. But, I wouldn't replace the GPU card yet.
Given the age of your system I suspect your is quite dirty inside and needs a good cleaning and the thermal paste on the CPU & GPU needs to be replaced.
I would start there cleaning out the dust and redoing the thermal paste. It just maybe the issue is heat. To help monitor things I would also install this app: [http://www.tunabellysoftware.com/tgpro/|Temperature Gauge Pro] so you can see the before & after conditions.