Why does my computer restart periodically?

Question

Computers

The client has a computer. Started rebooting from time to time. They sinned on the power supply, the replacement did not save. Put the UPS, nothing has changed.
We set up a test computer, the employee worked fine on it. There were no reboots. The client's computer was taken for a test, the computer was running various tests for a week, there was not a single reboot. Delivered again to the client, the computer began to reboot. Further more interesting. We bought a new computer, we put this employee, he again began to reboot periodically. What could it be?
Despite the fact that the test computer worked fine.
I'm starting to believe in poltergeists. :)

Answer 1

It is logical to sin on power or a monitor, it is logical to install an uninterruptible power supply and change the monitor.

Answer 2

It's not a joke, but it's definitely out of the question, isn't it?

Answer 3

It will be necessary to try to ground the employee :)

Answer 4

This, of course, is not a computer, but my router constantly hung / rebooted when Wi-Fi was turned on on it. And when the installers of the provider came, they never hung. I even contacted the manufacturer, their engineer connected remotely and also could not understand anything.
As it turned out later, this happened strictly at one specific point in space on the table (where he usually stood). Shifting it by 10 cm corrected the situation. Even bending one of the antennas to a different angle helped. How long did it take to figure it out ...
In general, check the electromagnetic compatibility.