After assembly, the computer turns on, turns off for 1-2 seconds and turns on again. What to do?

Question

Iron

Yesterday I assembled a computer, but I was expecting a problem at the end.
When you turn it on from the button on the case, the computer turns on, the cooler starts spinning, it immediately turns off for 1-2 seconds, then it turns on again and everything works fine. In the bios, the temperature of the processor shows 30 degrees.
The front is connected correctly, everything is drawn in detail on the motherboard.
Cooler install Deepcool Theta 15 PWM. Screwed tight, no gaps.

Answer 1

I think at first there is a self-diagnosis, so the cooler spins and stops, and only then it is fully turned on.

Answer 2

Bios memory tests
It was once on a laptop when I added a bar

Answer 3

Check your memory.
My computer simply did not start with some dice. I had to select by poking.
motherboard is gigabat.