Windows 10: Pc suddenly restarting while under load. BSOD only rarely.

Hello Microsoft community, Im a bit desperate for some outside opinions on what the issue with my pc could be. First off specs: Asus rog strix b360-f gaming, i9 9900kf, evga 3060ti xc, 48 gigs trident z, 2 2.5 ssd, 1hdd, 1 m.2, brand new TT 750w golf PSU a week old. Onto the issue, which I am aware is common, but extremely hard to pinpointgreat. Basically maybe a week or two ago my pc started restating under load spontaneously. Jumping the gun and assuming it was a wattage issue with some of the new hardware i put in, i swapped the psu to the 750w one which should be more than enough fo

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Random Restarts, No BSOD

He mentioned that he had done that above. Also I provided a BSOD details app. There were no BSODs to report.

 

Random Restarts, No BSOD

Reboot and hit F8 to get into the Windows Diagnostic Settings before Windows boots. Turn off "Automatic Restart on System Failure." This way we can completely rule out BSODs. Sometimes a PC will BSOD but the PC will restart so quickly that the BSOD is not even rendered.

 

PC reboots (no BSOD) on CPU stress

Hi fellow TPU members,

not too long ago, the box my gf's PC was transported in gave way on the stairs. Yes, you got it right, that sent a PC tumbling down a flight of steps alright.

Stuff broke. Also, stuff remained intact somehow.

I bought her a new case, new CPU cooler and we removed the floppy drive, so far so good. But the PC started crashing for no apparent reason, without a BSOD.

I tested:

• Memtest86+, left overnight, 0 errors
• Scanned the HDD for errors, nothing there
• Left FurMark burning up the GFX card and drawing some power, no problems there
• Ran Ubuntu from a Live CD doing nothing special, didn't crash
• Ran OCCT, Linpack, Prime95, all 3 caused reboots within minutes
• Oh, and I disabled "Restart on error" so I'm sure it's not a BSOD.

Now, I can still try and test something CPU intensive on Ubuntu, but my guess is the CPU didn't survive.
Before I do anything drastic like whippin' out the magic wallet and get her a new AMD quad, is there anything I didn't see?
I'm guessing the motherboard could be borkered as well, but can I check that other than replacing the CPU with a working one?
Any suggestions on which proggies I can use from an Ubuntu Live CD to stress the CPU?