iMac shuts down, fails to boot up.

Hello iFixit

My iMac is experiencing an issue, I don't know what the cause is. It is an Early 2008 with 3,06 GHz iMac.

The iMac shuts itself down after some time it's on (5-15 minutes) it looks like it just cut the power almost as if someone is unplugging it - I tried it also with different power cords and outlets. Than boot up again and after 30 seconds again shuts itself down, than again and then no boot chime or video.

Than I removed the iMac's glass screen and front shell to find the 4 diagnostic LEDs. The first one lit always. When iMac is able to but up first and second LED go on, than following by LED 3 and then LED 4 - and it boots up. When it not able to but up, the LEDs get form state where LED 1 is powered to state where the first 3 LEDs are powered on - I can hear the HDD, but none of the fans is moving.

I tried to change the HDD, than detached the ODD, nothing is helping. I tried it to boot up without the LCD attached and it was the same, first it managed to boot up (heard the boot chime and was able to connect to the iMac via TeamViewer) and after some time (5-15 minutes) again shuts itself down, then again the three LEDs came and only the HDD was moving.

I also recorded an video, if it helps to find what is causing the problem (sorry for bad video quality): https://youtu.be/gNfwILC2rNw

In the 15 minutes able to operate I also tired to stress test the CPU and GPU with the CINEBENCH R15, both (GPU and CPU) finished successfully. Than monitored the temps with iStat Menus, the max temp was from the GPU with 54 °C (129.2 F), what I don't think the temps are the problem.

I really don't know what to do next. If someone had the same problems, managed/knows how to solve it, please help :)