What can cause a PC to restart spontaneously?

Question

Computers

Hi all! The question is, the PC constantly reboots, I just can’t understand what is the reason. I reinstalled the system, cleaned it from dust, changed the power supply (it costs 650, changed it to 450), changed the RAM. The only thing that did not touch this video card and processor.
The PC can reboot both after an hour of use, and when turned on, after that again and again. In general, it is not clear what the reason is.
Stress tests checked for performance, everything is fine. Temperatures are normal, results are normal.