XPS 8960 in boot loop

Has been doing this for some weeks....At power on, system fails to boot...Dell log, 9 cycles of spinning dots and then CRASH!...Does that again, once or twice and then loads System Assist menu....and then the machine boots normally. The Time of Day will now be wrong but everything else is where it should be.

I have...
-Replaced the CMOS battery
-Reset BIOS
-Attempted a System Reset installation

and just about everthing else but a full system install of a new copy of Windows 11.

Disk1 is a 4TB NVME.
Disk2 a 1TB SSD
Disk3 a 2TB HD

System passes the Diagnstics every time. 

My suspicion is the BIOS and boot sector are being clobbered at shutdown or start up. ...but how? And how to nail the cause down for certain. I'd rather know that a full Installation will solve the issue rather than do one and end up with this unresolved.

Anyone run into this exact pattern? Push start; live through 2-3 boot attempts; Support Assist diags - about 5 minutes; system starts and runs fine.