External monitor goes black on XPS 15 9550

Whenever I run an external monitor (only) on the laptop's HDMI connector or a VGA adapter on the USB-C connector, the screen regularly goes black for a few seconds and the speaker beeps. Sometimes, not often, the resolution of the external screen will have changed when the monitor comes back from black.

I just updated the BIOS, chipset, and video drivers from the Dell download site.

The external monitor is an ASUS VW266H.