Do you have a

00:1f.6 Ethernet controller: Intel Corporation Ethernet Connection (7) I219-LM (rev 10)

NIC?

Is your interface switching between down and up every few seconds?

Disabling Energy-Efficient Ethernet helped me.

Create a new unit systemctl edit --full --force disable-eee.service with this content (if the interace in question is called eno1):

[Unit]
Description=disable EEE for eno1
After=network-online.target

[Service]
Type=oneshot
ExecStart=/usr/sbin/ethtool --set-eee eno1 eee off

[Install]
WantedBy=multi-user.target

Enable it with systemctl enable disable-eee.service, reboot and check with

$ ethtool --show-eee eno1
EEE settings for eno1:
    EEE status: disabled
    ...