Generally that is a hardware conflict or defective hardware. Since the HDD and MB have been replaced either a RAM issue (test it with Mem86) or a CPU issue. And by process of elimination it sure starting to look like the CPU. CPU problems are rare until it is a CPU error. I would think the Dell tech should be bringing another CPU to test the system and if that is the problem you will know in short order.
Hi,   We have a refurbished ~1.5 year old XE3 in original shipped configuration (i7 8700, 2x 500GB HDDs, 16 GB DDR4, WIN10) that will not boot into Windows 10.  The system is still under Dell Support contract. The HHDs were wiped by the previous owner. Created a bootable USB Windows 10 installer, the Windows 10 installation failed.  ("WHEA Uncorrectable Error") Contacted Dell Support, conducted extended troubleshooting and system tests, no failures found. Dell sent out a replacement HDD with the factory shipped software image. Installed the new HDD, removed the second HDD, so only one factory imaged drive in system. Windows 10 would not boot, or repair.   (each time >  "WHEA Uncorrectable Error") Contacted Dell Support, conducted more diagnostics, troubleshooting, and resets.  No failures found. Each time we attempted install or repair we get  "WHEA Uncorrectable Error". Dell sent out a technician with a motherboard. Replaced the original motherboard.  Moved the original CPU to the new motherboard.  When he tried to boot the system "WHEA Uncorrectable Error" again. At this point the CPU is the only original system logic component. (Still using the original case, PS, fan assembly, ..)   Anyone have any ideas as to what the problem may be?
Optiplex XE3 - Windows 10 will not boot. WIN10 Repair and Installation also 
fail. "WHEA Uncorrectable Error"