How prioritize ethernet over wifi in Ubuntu 18.04?

Open the terminal application and type:

nm-connection-editor

On the window that opens, select your Wired Connection and click the gear icon at the bottom of the window to edit it. You need to select the General tab, enable "Connect automatically with priority", and change the priority to 1. This will prioritize the ethernet connection.

Understand that:

• Ubuntu normally auto-selects the connection with the best performance.

• If you have an ethernet connectivity problem, that would explain why your connections keep on switching.

• Check that your USB-C port doesn't power itself down at idle... you may have to check BIOS settings for that.

It was hardware: my Toshiba USB-C Travel adaptor (PA5272U-2PRP). The problem persisted many weeks in which I worked mostly on wifi. After much hesitation (expense) I bought a similar adaptor: Goobay USB-C Premium Multiport dock. It works flawlessly.