I had a similar problem with my rig recently. It would instantly restart in-game, although at least a few things might be different. It was really only happening in 1 game for me as far as I can tell, and it was only in 4k resolution. My voltages, temps, everything was normal at the time of the restarts and it never restarted in 3dMark, only in Far Cry 5.
Since it costs nothing but time, I can suggest to you to try a software solution first before you spend any money to replace the PSU. First, just make sure your voltages (3.3v,5v,12v) are ok as Scout suggested. If they're good, you should uninstall the GPU drivers and Geforce Experience using DDU in Safe Mode while NOT connected to the internet and then install your drivers again from a pre-downloaded installer file for version 431.60 which is an older and presumably more stable driver than the recent ones. To paraphrase the advice given to me over at the nVidia forums (Looks like a lot, but I've just been very specific about each step):
1) Download the driver:
http://us.download.nvidia.com/Windows/431.60/431.60-desktop-win10-64bit-international-whql.exe
2) Download DDU:
https://www.wagnardsoft.com/content/display-driver-uninstaller-ddu-v18019-released
3) Disconnect Network Cable / Disable WIFI adapter
4) Run msconfig by typing it into the Windows start menu. click "Boot" tab, check "Safe Boot" option then "Apply" and reboot your PC.
5) Install and run DDU and select "Options" from the program's menu in the upper left.
6) Check ALL the boxes for nVidia as shown below:
NOTE: the warning about C:\NVIDIA sounds worse than it is, but it means that DDU will delete the folder where downloads of previous nVidia driver install files (like the one you manually downloaded in step 1) have accumulated. If you have a slow internet connection and you would like to keep these files just in case, then don't check this box. I just got rid of them all myself.
7) Click "Close" in the options menu, select "nVidia" where is says "Selected Graphics Driver" and then click "Clean and do NOT restart"
8) Open msconfig again and make sure Safe Boot is unchecked (uncheck and Apply if necessary) and then reboot your PC.
9) Now out of Safe Mode and back in normal Windows mode, find the nVidia driver 431.60 file that you downloaded and start the install process. Select CUSTOM and make sure ONLY "nVidia Driver" and "nVidia NGX" are checked (do NOT install PhysX, audio drivers, Geforce Experience, or anything else at this time). Now, check the "Clean Install" box, finish the installation, and then reboot your PC.
10) Reconnect Network Cable / Re-enable WIFI
11) If using any of these, remove them:
Uninstall iCue | Link
Uninstall CAM
If using a PCIe Riser, remove it.
12) Retest with games/software that are giving you restarts.
13) If everything is ok by now and you're not getting restarts anymore, then open the nVidia driver installer again and install PhysX and retest. If still ok, add the audio driver and test again. If one of these items brings the problem back, start over from step 1 and leave out that item. The only thing I added to my installation was PhysX. I think I'm going to do without Geforce Experience for now. I might try installing the Audio driver, but haven't yet.
For reference, here's the link to my question over on the Geforce forums, with greybear's responses: