mayatola
Just to reiterate, it only appears to shut down at night (and recently, since it started getting cold). Would anyone know of a reason why that could be? At first it used to happen anytime after 9pm, but now it is only happening around midnight. Anyway, the only other thing I can think of that might have any effect is that I lowered the voltage of the CPU from 1.45v (default super high Ryzen voltage) to 1.4v. This has never caused any problems in the past, but I dunno, I'm sure one of you guys knows better than me.

aka_STEVE_b
Aren't there rolling brown outs in California ?  -  could be the problem
 
You are saying it's only at night. .. which could also be the power grid in your area gets overstressed when everyone in the area comes home from work & starts powering up everything. 
That would register as a power fluctuation & some units flash off as a protection of too high or too low of a reading  in voltage.
 
Are you guys plugged into the same breaker line from your house. the draw of 2 units could be too much load on the house line and one or the other unit has to give .
 
..or it could be as simple as the psu is going bad / getting weak from use or age .
 
Good luck

mayatola
So stupid me, I only plugged in the single 8 pin EPS when there is actually 8 plus another 4 pin socket.

mayatola
Thanks so much for all of the suggestions!
 
As for the shutdowns only at night, it is only happening during gaming, but it was only happening while gaming late at night. But yesterday, it did happen earlier than 9pm or midnight (happened around 7pm). We don't have any heater kicking in at night or anything either, but the problem seemed to go away for a while when we changed to a different outlet (which is also on another breaker, our electrical wiring is very odd in our old house, it's also aluminum, not copper).
 
I will try to measure the 12v rail as that isn't something I even looked at yet. One thing I forgot to mention is that I manually set his voltage on his CPU because I heard that X570 boards have the voltage set too high by default (his was 1.45v). I set it to 1.4v but I set it manually, not as an offset, and I didn't touch LLC. I'm not so much of an expert on this stuff but I think that might be a problem, so I reverted the CPU voltage back to default, and so far, the certain situations that cause the shutdown has stopped. I guess the next few days to a week will tell if that was the problem, but I will check the 12v rail in the meantime as well to see if it isn't the PSU.
 
As for that extra 4 pin, yeah, I don't know why it seemed to have stopped the shutdowns for a while. Yes, I know that it shouldn't be necessary for stock operations, especially for the 3700X, but now it doesn't seem to matter as they started happening again. The shutdowns used to happen (for other games like The Division 2) when he tried to start up the game, but now it only happens in Cyberpunk, and only after sitting in the inventory/crafting menu for longer than a few minutes and then trying to get back in game. *edit: checked the 12v and it is 12.096v, so I guess it's ok. The PSU is pretty new (purchased around 4-5 months ago).
 
I will give another update, but I think you all so much for your support! Hope you guys have a safe and happy New Year!

• Make sure DOCH is enabled. Some XMP RAM (DOCH for AMD) will not function properly if it's not configured with the XMP profile that provides a higher voltage and timings it was stress tested on.
• Make sure you have the latest BIOS installed on the motherboard, there might be some stability fixes.
• Check your hard drives. I've had a faulty platter drive cause shut downs, also not sure the age of the SSD but solid state drives have a read/write capacity. Like my Samsung 970 EVO 1TB has a 600TBW rating, which means after 600 terabytes of data has been written to the drive, it's considered end of life by the manufacture.
• Make sure the motherboard is properly seated on the case stand-offs.