As old as your iMac is, have you cleaned out the dust? Dust buildup can lead to over heating issues. Remove any and all things plugged into it including the power cord, Remove the RAM access grille. Vacuum all openings starting with the RAM access area (air intake). Vacuum all ports and plugs, DVD slot and the opening across the top of the rear of the body. Blow compressed air through all your openings and vacuum again to remove any dust you loosend. Reinstall the RAM grille. Plug in the keyboard and mouse if not blue tooth. Insert power cord...
You are now in a perfect position to do a
- Shut down the computer.
- Unplug the computer's power cord and ALL peripherals.
- Wait 15 seconds.
- Attach the computers power cable.
- Wait another 5 seconds and press the power button to turn on the computer.
- It is the 5 second timing that initiates the reset.
Here is a link to a Widget to monitor your temps and other operations: