Similar issue - Surface Laptop 2 connected to 2 external monitors via surface dock 1. works fine for hrs, or even a full day - then for a period of time it is as if someone is unplugging & plugging back in the monitors over & over again. Could be just one or two resets in a row - could happen continuously for 15 minutes during which the device is unusable - just have to unplug. Unrelated to graphics load or how much is running. unrelated to if my laptop is open or closed.
Thanks to a previous response here I can see what the issue actually is - looking in Events view under Errors I can see the device goes through periods of constantly trying to update the firmware on the dock, which results in a quick reset of the dock. I have tried manually re-updating the firmware but error still occurs.
Recomend checking event viewer & seeing if anyone else sees the same error logs in event viewer.
Unfortunatley I do not know how to stop windows form trying to update the firmware yet - but will update if I find a way. If anyone knows how please shout.