PC built in July 2020 with the following specs – all components were brand new:

• MoBo:  Gigabyte Z390 Aorus Ultra  (Note: Also tried with Asus Maximus XI Hero – see info below)
• CPU:  Intel i9-9900K LGA 1151
• PSU:  CORSAIR HX Series HX1200 CP-9020140-NA 1200W
• Memory:  Patriot Viper Steel DDR-4400  -  32GB  (8GB x 4)
• M.2:  (2) Seagate Firecuda 2TB
• SSD:  (2) Samsung 860 QVO 4TB
• Optical Drives:
  • LG Black 16X BD Burner incl BluRay 3D  (WH16NS40)
  • LG Black 12X BD Burner (UHD  4K)  (WH16NS60)

Started experiencing issues in October where it would occasionally crash and reboot while playing Raft and other games.  Updated Windows, all systems drivers, and NVIDIA Drivers.  Things gradually became worse – more frequent crashes / reboots both while playing games (frequently), and other random times (less frequently).  I’ve tried the following, many multiple times with various different iterations:

• Attempting to Repair Windows (no luck)
• Ran Anti-Virus, Malware detection, Spyware detection
• Updating to differing drivers (including NVIDIA drivers)
• Added Steam as AntiVirus Exception
• Disabled Hyper-V as Windows Feature, and as Services
• Disabled Intel Virtualization Technology (VT-d) in BIOS
• Clean Boot
• Disabled Anti-Virus
• Memory Check and MemTest86  (both run – MemTest86 twice – no issues with memory)
• Updated Minimum CPU Power Settings
• Reinstalled Windows (clean install) and tried reverting back to drivers and BIOS that were available when I first built the system in July, 2020
• Ran Display Driver Uninstaller in order to install an older, known working version of Graphics Driver without risk of Windows reinstalling a driver without my consent
• Ran Driver Verifier
• Another ~20 different Windows 10 reinstalls
  • All clean installs, not reinstalling over old version of Windows
  • With and without Windows platform and/or Security updates
  • On differing BIOS versions
  • Using differing drivers for the Intel Management Engine, Graphics Card, RST, SATA, INF, and other components
  • Eventually, system got to the point where it looped through crash / reboots.  Could not even logon.

Due to motherboard issues (after another BIOS update, system would not POST even resetting CMOS), I purchased a replacement Motherboard (ASUS ROG Maximus XI Hero)

• Within minutes it was evident the issues were recurring (random reboot before updating drivers).
• Tried to run ASUS driver update tool.  Mid-way, the system crashed/shutdown.
• Reinstalled Windows from scratch (clean install and tried again.  Same thing.
• Cannot say for certain if the Graphics drivers were being installed, but after restart…
  • As soon as I started typing password to login, the system crashed / shut itself down
  • This happened two or three times
  • Now, it is back to looped-reboots where, as soon as Windows loads, the system restarts itself
  • Removed the card and now using the built-in VGA mode so that I can access the system with stability

 
Questions:

• Is there anything else I should try testing / verifying?
• Should I try decreasing the overclocking that is built into the “Ultra”?  (Disappointing to have to do so).  If so, can you share a link to guidance re: how to do so?
• Is there any value in taking apart the graphics card and re-applying thermal paste / thermal pads?
• Any other thoughts at all???

Hi USBARLOW - Thank so much for your insight!  You are right: Gigabyte Z390 did have the NVME slots right under the PCIe slot I was using for the Video Card.  However, the motherboard I switched to (ASUS ROG Maximus XI) does not have the NVME slots near the video card.
 
Unfortunately, even if I wanted to try a different video card I can't get my hands on one for less than $1000 due to the shortage.
 
Another odd thing I've found is that, with the most updated NVIDIA drivers, I can't even run the PC long enough to get hot before it crashes / reboots (if I can even login to Windows before it does so).  Going to take apart the video card this weekend to re-apply thermal paste, then try reinstalling with decreased clock speeds to see if that helps.  Fingers-crossed!

My 2080 always has 1 fan spinning at a low rpm no matter what.
 
Try removing the card and update the drivers using the integrated gpu.
Try a lower PCI-E slot just to test not for long term use.
Check power leads to the power supply.

After manually setting the fans to run, everything seems to be a-ok (set them to 70%). Cannot believe I never checked this before, although this is odd as I've never had to manually set fan speeds for any other video card. Will continue testing for the next week to be certain.

Thanks very much for your posts / suggestions!