Dell Latitude 7480 Laptop that freezes up

We have a Dell Latitude 7480 Laptop that freezes up.  The computer can just be running but sitting unused or in the midst of heavy processing.  The screen is locked up to the last image, but the keyboard, mouse-pad, and external mouse (if being used) have no impact on the Laptop.     Letting it sit while locked up has no impact on it.  We can power it down with the power button and that unlocks it for a while.  Dell onsite support could not fix it, and two returns to the Dell repair center did not help despite replacing drives and new OS (Windows 10).  We just had Dell send out a tech who replaced the motherboard, memory, and Drive.  It froze up on us twice within hours.  We and Dell seem to be stumped.

We just performed the full ePSA test several times with no findings.           BIOS = 1.9.3          EPSA: Build 4304.17    UEFI ROM.  Has anyone had any similar issues.