SOLUTION. Well, i gave the PC to an store to fix it, as i couldnt figure out what's happening. PSU works fine, so does the HDD and SSD hard drives i have. Now it's clear i think why the problem was so complicated. BOTH the motherboard AND the RAM had problems. Replaced the RAM with a new one, and now what happens is that when i turn on the PC, it starts, then you hear the fans shut down, then turning up again 2 or 3 times. (ITS NOT THE PSU. the guys in the store tested my machine with brand new PSUs and the same stuff kept on going.) The reason why pc restarts on its own, is that there is something wrong with the motherboard. Something that makes tha motherboard not able to boot. The motherboard understands it cannot boot, and then reboots, until it will finally boot into my OS.
So it was a combination of 2 major failures of the hardware..
Thanks in advance, i hope this thread will help others with similar problems, and next week i will replace my motherboard, and leave a comment here to make the thread completely solved!