Maybe total video bandwidth issues? Working past thermal limits where there wasn't adequate testing of drivers? Cheap external 4k display? Cheap external adapters from USB-C to DP and/or HDMI (both seemed to create problems).
no time to root cause, solved the problem short term and moved on.
update: 2/11/19, stable after one week by going to the 2 external+laptop display vs. the 3 external.