Z640 boot issue: works with bootloader but not Win Boot Manager

[hiebertw07]

I have a Z640 that is having trouble booting with Windows Boot Manager, but boots perfectly well when I go into BIOS and boot from the EFI/bootloader file. If I try to use Boot Manager, the screen goes black right after the HP splash and the machine is unresponsive and has to be hard reset. 

BIOS and drivers are updated, anything relating to safe boot in the BIOS has been disabled, I've cleared CMOS, I've reinstalled Windows twice, I've removed & reseated all PCIe devices, and I've run just about every diagnostic tool that I could find. 

I'm using all OE parts aside from an m.2 boot drive on a PCIe adapter. I can see the SSD in the UEFI boot menu, so I don't think that's the issue. I'm not sure if I'm missing some setting or need to map WBM (is that a thing?). Heck, I'm not even sure if it's a BIOS issue or a Windows issue. Either way, I've been troubleshooting and searching for hours and would definitely appreciate some help.

Specs: Win 10 Pro 64-bit on Samsung 870 Pro NVMe. E5 2667v4. Quadro P2200. BIOS version 2.56.  

[DGroves]

set the bios boot delay time to a larger value