Peculiar problem with Precision 3650 and graphics adapter

I have two Precision 3650 towers here that I am rebuilding. I found a curious problem affecting both systems exhibit identical symptom. The processor is a supported Xeon W-1250 (SRH48).

Using the iGPU Intel processor graphics (via onboard DP port), the system will spontaneously reset/reboot shortly after loading to the desktop. Usually within 90 seconds but has gone up to about 5 minutes. There is no crash dump or logs other than the 'Windows did not shut down properly' dirty bit error that gets reported on reboot. It does not happen early in the installation of Windows but it does happen very late (when Windows is guiding you through SETUP questions)

When a graphics card (dGPU) is installed in the PCI-E x16 (PEG) slot, the system is 100% stable. But wait, there's more! It is 100% stable even when I am actually connected through the integrated graphics. i.e. no display connected to the dGPU. I noticed some of the benchmark apps were still defaulting to the dGPU for some workloads and outputting via the iGPU. So I disabled the dGPU in Device Manager, restarted and set primary display adapter in BIOS to Onboard, booted to Windows, checked the dGPU was still disabled, to ensure the iGPU (Intel processor integrated) would be used for everything.

I ran PCMark, 3DMark, processor stress, memory stress for several hours (cumulative), 100% stable. I confirmed the iGPU was being utilized for ALL workloads by using Task Manager and watching utilization of the various resources such as 3D, Video Decode, Video Encode, or Memory Copy. All showing utilization on the iGPU not the dGPU.

The dGPU I have been testing are both slot powered (under 60W board power), no aux power connector. Running all the benchmarks and stress tests on the (enabled) dGPU also is stable, BTW.

Simple recap:

System with only IGP = spontaneous reset/reboot

System with dGPU inserted into PCI-E x16 (PEG) whether utilized or not = 100% stable via IGP (or dGPU)

Windows 10 (22H2) and 11 (24H2) exhibit same problem. Latest drivers or older drivers, same problem. Latest BIOS or the oldest BIOS I am permitted to revert to several versions ago, same problem. Tried all three BIOS settings for primary display adapter, Auto, Onboard, or dGPU. Multi-display support on or off. PCI-E (PEG) bifurcation Auto or x16. Disabled almost all onboard devices except for LAN and rear panel USB. Same problem. Kernel DMA protection in Windows ON or OFF, same problem. Core Isolation/Memory Integrity OFF. ReBAR OFF (always a safe choice). PCI decode above 4G ON or OFF, same problem.

Any ideas?