Has any hardware changed in the system in the last few weeks? I would be looking to see if a windows update is available or try rolling back to a previous NVIDIA driver to see if the issue still happens.

So, no Blue Screen, just reboots your PC?
 
please list all hardware & OS
 
Fast boot off in MB BIOS?
 
Fast Startup off in Windows? 
 
Try holding shift key down, then in Windows select shutdown, Hold Shift key until all power is off on PC.  This clears the Windows startup file, which Windows uses by default instead of polling your hardware & drivers for changes
 
Any monitoring Software running? case fans, CPU, GPU, temps ?

Make sure you are offline when you uninstall or clean install your GPU driver (that you previously downloaded) & turn off all background software, Yep even security software.  Now install Nvidia driver, use clean install.  Shutdown PC with the Shift key held down.  Reboot, check security software is running, then enable internet
 
 

That is often actually a blue screen but the error occurred in the graphics stack so no screen could be displayed. Download Whocrashed and run it, you may have a bunch of crash dumps from the bsods. 

PSU is probably 2 years old. bought it exactly when i got the 3080. one of the evga 750w gold rated.

I doubt its that. It only crashes in the first few mins of powering on my pc and sometimes when starting up a game or web browsing. But once im in the game (in game so pc is using more power) it doesnt crash.