I'm using an official Apple thunderbolt-to-ethernet adapter and also connecting to an external monitor via the other thunderbolt port. I'm using a MacBook Pro, late 2013, with and updated Mojave.
When I use the ethernet, there is glitching on the screen, about 8-10 times an hour, where large black rectangles full of noise flicker briefly then vanish. If I disconnect the adapter and use Wi-Fi, this does not happen.
What is causing this?