Ubuntu 18.04 reboot when shutdown

How do you initiate the shutdown? With a mouseclick on the related buttons in the top right corner of you desktop, or with the shutdown command, or by pressing the power button of your system, or ... ?

For diagnostic purposes please provide the output that you receive for the commands (to be executed in a terminal window)

uname -a
lsb_release -crid
sudo dmidecode|grep -E 'anufact|roduct|erial|elease'

What happens if you try shutting the system down with the command
sudo poweroff
Does it switch off, or does it reboot?

I did it by right corner button and poweroff command and from time to time my computer reboot

lsb_release -crid

                     Distributor ID: Ubuntu
                     Description: Ubuntu 18.04.4 LTS
                     Release: 18.04
                     Codename: bionic

uname -a

                    Linux comp 5.3.0-40-generic #32~18.04.1-Ubuntu SMP Mon Feb 3 14:05:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

sudo dmidecode|grep -E 'anufact|roduct|erial|elease'

 Release Date: 08/21/2012
        Serial services are supported (int 14h)
 Manufacturer: Gigabyte Technology Co., Ltd.
 Product Name: To be filled by O.E.M.
 Serial Number: To be filled by O.E.M.
 Manufacturer: Gigabyte Technology Co., Ltd.
 Product Name: H77-DS3H
 Serial Number: To be filled by O.E.M.
 Manufacturer: Gigabyte Technology Co., Ltd.
 Serial Number: To Be Filled By O.E.M.
 Port Type: Serial Port 16550A Compatible
 Manufacturer: To Be Filled By O.E.M.
 Serial Number: To Be Filled By O.E.M.
 Manufacturer: 029E
 Serial Number: 00000000
 Manufacturer: 029E
 Serial Number: 00000000
 Manufacturer: Intel
 Serial Number: Not Specified
 Manufacturer: Kingston
 Serial Number: 1510AE71
 Manufacturer: [Empty]
 Serial Number: [Empty]

"from time to time my computer reboot"

How often does it reboot instead of shutdown every time, or only seldom? You mentioned your network card, what kind of network connection is it?

"You mentioned your network card, what kind of network connection is it?"

Ordinary LAN connection. I mentioned because when i unplug LAN cable from network card all shutdown correctly work

"How often does it reboot instead of shutdown every time, or only seldom?"

 very randomly , today i shutdown my comp 6 times and 3 times was reboot instead shutdown

I am sorry, no idea. Maybe somebody else can suggest something.

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