I'm having the same issue with my 3080 FTW3 Ultra.  I have a 850w GA 80+G MF ATX I just bought.  My CPU is an i9-10850k which is currently overlocked.  
 
I bumped the gpu down 50MHz in afterburner and it seemed to work on initial testing last night.  Was able to play Assassin's Creed for 20 mins without a crash.
 
Will do more testing tonight after work.
 
I'm not opposed to switching my 850 for a 1000 if that is the actual problem.

The corsair 850RM I have seems to have no issue with these cards.   Look into an 850 watt psu

I am also having problem exactly like this for my 3090FTW3 Ultra. I have a Seasonic Titanium 850Watt PSU. It only happens when I overclock with power slider maxed out to 107%, and so far it only happens in Metro Exodus. All benchmark seems fine.
 
Symptom is just as many has described: PC suddenly turns off, then reboots. In event manager it shows Event ID 41, Kernel-Power 63.
 
So what, I am going to need a 1000Watt PSU to power the 3090 now?? My UPS does show the at heavy GPU duty the system draws around 700Watt

I have the 3090 FTW3 Ultra as well.  OC Clock +100, OC Mem +750.
 
I have a 1000 Corsair PSU.  On my APC, it has an option to show current wattage.  Doing a benchmark, I see it cross the 900 line fairly often.  I've not seen it go above 907.  
 
Without OC, the highest I've seen it hit was 793.  
 
I personally would reccomend the new Evga 1300 PSU they just released.  That's my plan for when I upgrade either when Zen 3 numbers come out, or in March when intel numbers come out.
 
Running 9900KS @ 5.3 as well.

So I'm ****ed with the corsair ax850 I got 2 months ago? 🤤

fofal
So I'm ****ed with the corsair ax850 I got 2 months ago? 🤤

I'm in the same boat, but I'm waiting for more answers before running out to get a 1000w.  Maybe it is something that can be fixed with a firmware update.  Everything I read prior to buying and building my PC said 850 should be more than enough.  

Wow that's crazy. I'm going to have to upgrade my psu it sounds like. My new case can take two psu's so maybe I should just get a 650 for the cpu and dedicate my platinum 850 to the gpu.

mlaningham
I have the 3090 FTW3 Ultra as well.  OC Clock +100, OC Mem +750.
 
I have a 1000 Corsair PSU.  On my APC, it has an option to show current wattage.  Doing a benchmark, I see it cross the 900 line fairly often.  I've not seen it go above 907.  
 
Without OC, the highest I've seen it hit was 793.  
 
I personally would reccomend the new Evga 1300 PSU they just released.  That's my plan for when I upgrade either when Zen 3 numbers come out, or in March when intel numbers come out.
 
Running 9900KS @ 5.3 as well.

If I overclock my VRAM to +1000 on my 3090 FTW3 with no 3D load at all, my computer reboots itself - on a EVGA 1600 watt T2 power supply.

So, you're likely pushing your card too far for the game (and if you enabled RTX in Metro Exodus, the game is using parts of the GPU that are otherwise turned off for non-RTX games and benchmarks - which means stability testing with RTX is needed). You could verify this by lowering the CPU overclock and testing the GPU again with the same game/OC.

My computer actually hasn't had any issues with the Corsair 1000 and the OC's I have running.  I was just attempting to help OP see that he might need a higher wattage PSU.

mlaningham
My computer actually hasn't had any issues with the Corsair 1000 and the OC's I have running.  I was just attempting to help OP see that he might need a higher wattage PSU.

Whats the power draw when running Prime95? Curious how much your cpu is munching.