iMac shuts down randomly and suddenly
I have an sudden turn off issue with my iMac 24" Early 2009, like on a power outage. First, I cleaned the fans and replaced the thermal compound in order to exclude possible thermal issues. After that the temperatures decreased from 60 degrees Celsius to 35 degrees Celsius, which is a very good deal. Unfortunately, the device keeps random shutting down itself albeit the temperatures are lower than on any other device I have.
Strangely you can let it stay in idle the entire night or install an OS or perform a memory check and it will run stable without shut off but after starting anything on the web e.g. a Youtube Video or launch a web site the system will get off within 5 minutes.
I replaced the Delta power supply with a new one from LiteOn, but this hasn't solved my issue. I also have taken a look on the four LEDs on the right side of the logic board. When the sudden power off takes place, three of the four LEDs cease to light up. Only the first LED from left stays on every time. Also I can turn the iMac on again immediately after shut down at every time.
My closest assumption is that the graphics card might have an issue but I think that this kind of error would more likely lead to freezing screen or kernel panics instead of sudden power off.