P2422HE, dropping power and restarting

This behaviour has occurs with two different laptops (HP and Dell). Both fully updated, etc. I'll use the USB Type-C with power delivery. The monitor also has a ring light plugged in as well as a USB lead which gets used for various things. 

When I have extended screen the monitor will, about once an hour or so, lose image. The laptop will make a "device unplugged" sound. Anything powered through the monitor will lose power (ring light goes out and into standby). Laptop freaks out and flashes black screen to reset away from extended screen. Then within .5 a second or so the monitor will show a "no signal on USB Type-C message" before getting the signal. The laptop will then blackscreen again to extend onto the monitor and everything is good (except any USB devices need to be taken out of standby as they lost power from monitor).

It's not the laptop: two different makes used. Not drivers, I'm fully updated. Not the stuff plugged in, as far as I can tell. The only possible pattern is that it hasn't yet happened when I am actively using the second screen (as in windows and pointer on there. That might be luck though as I only actively use it about 10% of the time.

Any pointers gratefully received.

Thanks