I have the same issue with my 3080 FTW3 Ultra as well. Running on an EVGA Supernova G2 850W that supposedly "high quality." I have been using this PSU for 5 years with my 6700K and 980ti and it was fine until now. GPU ran at stock speed and cpu has a slight OC at 4.5ghz while memory is running its default XMP profile.
Been dealing with restarts for about 2 weeks randomly. Most of the time, its under light load (youtube, twitch)...that's the sad part. Under stress test - no reboots. When gaming, it happened once... I believe Windows Event Viewer says that I have about 9 reboots in the past 14 days. I tried re-seating the cables, running it on stock, running an undervolt..and etc. Even monitored the 12V rail with HWinfo64 (fluctuating between 12.192 and 12.096V under load and a constant 12.228 idle).
Next stop is to try another replacement PSU. I am contacting customer support for this. Based on Nvidia's recommendation, the FE card is running at 320W and requires a 750W psu. By that logic/ratio …the FTW3 ultra card which runs at 380W - 405W (for me), would need around 900W to 967W PSU. But then again, a lot of people are running their 3080 fine on 750W and 850W. Well, done ranting...will continue to troubleshoot and good luck to everyone in the thread.