To all,
Mrtotes has a good point here. Download and run something like Temperature Monitor:
http://www.bresink.de/osx/TemperatureMonitor.html
That would be one of the first things to consider if a computer runs normally and suddenly shuts down. There was just a case here where the lower housing screws were loose. Here's the appropriate quote from the service manual:
"Replacement Note:
These torx screws must be tightened to at least 17 in.-lbs. If you do
not have a torque driver, you will have to make sure these screws are tightened by hand FIRMLY, BUT NOT FORCIBLY. Or, purchase the service tool (076-0899) in order to ensure the thermal pipe is firmly mated with the top base. If the bottom housing is not securely attached to the base in this fashion, the CPU may overheat and become damaged. For
more information, refer to “Thermal Paste Application’” in this chapter."
"Thermal Paste Application
Tools
This procedure requires the following tools:
• Plastic stylus or plastic spatula to remove the old thermal paste
• Plastic stylus or plastic spatula to spread the thermal paste
• Thermal paste (922-4757)
Thermal Paste Application
Procedure
1. Thoroughly clean the original thermal film from the mating surfaces (circled below) of the bottom housing and thermal pipe. Use a plastic stylus to scrape the surfaces clean.
Note:
Do not use an abrasive material or liquid cleaner.
2. Squeeze a thin film of thermal paste onto the mating surfaces of the thermal pipe." (there's been references here to Arctic Silver from RS and a later version that's better)
This would always be something to look for if you suspect or know the computer has ever been apart. These iMacs are not easy to get together correctly and the heat pipe routing is a definite "Gottcha!".
Richard