PC completely powers down randomly

[wizarduss]

My PC is randomly completely shutting down (as if it's a sudden power loss). This started when playing half life alyx so I thought that was overloading my system, but today it's also happened while the system was idling.

After one of these sudden shutdowns, I have to flip the switch on the PSU off and on before the PC boots again, and the first boot after it sounds like it starts up, then stops and then boots up as normal, though no motherboard beeping in that time.

 

I have checked the event logs, but all it says is there was an unexpected shutdown. 
I've also had memtest86 do 2 passes with 0 errors.
Temps seem to be ok as far as I've been able to see with the CPU getting up to 75 degrees and GPU up to 85-90.

 

My specs are:

• Windows 10 pro x64
• Asus Prime x370 pro (bios version 0612, default bios settings)
• Gigabyte GeForce GTX 1080 Ti Gaming OC 11G
• AMD Ryzen 7 1800x
• G.Skill Flare X F4-3200C14D-16GFX
• Samsung 850 PRO 256GB SSD (boot drive)
• Samsung 850 EVO 1TB
• Samsung HD103SJ 1TB
• WDC WD40EZRZ-00GXCB0 (4TB)
• WDC WD2003FZEX-00Z4SA0 (2TB)
• Corsair RM750x 80+ Gold (3,5 years old by now)