Why does the iMac screen go blank and restart after 5 minutes?

Question

iMac

Good day to all. They gave me an old iMac from 2008. Good old Intel Core 2 Duo, 4GB DDR2 RAM, etc. They gave it to me because the screen goes blank when the OS boots. I took it apart, cleaned it, replaced the old HDD with an SSD. Collected, rolled on it the latest supported OS (OS X El Capitan), and everything was fine. But, unfortunately, this defect recurred yesterday, and now, switching disks, alas, does not help.

I am not experienced in using apple computers / laptops, all I had and have is a phone and a watch from them. I would like to read the opinions of experienced users

PS: I read articles where they write that the problem is often due to RAM. I took out one bar at a time, still the same ...

PPS: the screen goes blank, and with it the fourth diode (LED4) goes out, which are located at the RAM slots. 5 minutes after the display turns off, the computer restarts.

PPPS: if you start from the installation flash drive, everything is the same

Thank you very much!