Actually it was a motherboard problem. Dell replaced the motherboard under warranty and I never had that problem again.
This happens even with Dell's Thunderbolt dock. Laptop would all of a sudden shutdown and this pattern blinks on the LED in front: 4 amber 1 white. Here's what it looks like [URL] Again, this happens even with Dell's Thunderbolt dock. In the past it would happen every once in a while, but in the last week it's intermittent to the point that I can no longer work. There seems to be no hardware problems, no driver issues - laptop works just fine when running on battery power or its Dell power adapter. But if I connect a Thunderbolt dock, or even a thunderbolt charger the same thing - shutdown after a few minutes and blinking LED.  The interesting thing is, when this happens, clicking the battery charge check button doesn't work either. It's as if the battery and all its control circuits have been shut down. I just bought this so that I can connect one simple cable and have everything neat & tidy LG 38WN95C-W 38" with 94W Thunderbolt 3 - and the laptop is upset!!! Is this a known problem?
Precision 5540 (2020 edition /w i9) keeps rebooting when connected to 
Thunderbolt docks and power adapters