iMac 24" EMC 2211 freezes after 10 minutes

The iMac freezes up after ten minutes of uptime like clockwork. It runs as expected under Safe Boot/Mode but after a regular boot, the freeze comes back.

I have replicated these conditions booting from an external hard drive running OS X.

I think the culprit is either the GPU (ATI2600) or the heatsink associated with it. I am not sure if an overheating CPU would exhibit this much uptime if it wasn't cooling properly.

I want to identify root cause by pointing an infrared thermometer at four places:

- the CPU

- the GPU

- the heatsink fins pointing to the CPU

- the heatsink fins pointing to the GPU

I hope to see CPU and CPU heatsink temps above room temp.

I actually want to see a very hot GPU and heatsink fins almost at room temp. This would indicate a coolant leak.

I can't find a guide for logic board removal. I am assuming this is necessary to remove and replace the GPU/heatsink assembly. I am not sure if I can take IR thermometer measurements while the iMac is running.

Any other thoughts and ideas are welcome.

Thanks for reading!