so now, your rig still shutting down randomly...?? because mine, i didn't notice any issue about that. and also, my PSU was Armageddon 750 watts. i change my original amd cpu fan to cooler master liquid lite 120...
my rig was this
Kingston 8Gb DDR4-2400x2
Amd Ryzen 1500x
Armageddon 750 watts PSU
Assuming this is not external..i.e. house power problems...
This is typical of a bad PSU or bad RAM/RAM settings
Since you have tried a second PSU already I'd look at RAM
Does this happen if you set RAM to BIOS defaults? For 2400 what dram voltage are you using?
It was automatically set to 1.2v, and I just set it manually to 1.35v in BIOS. G.Skill lists it as 1.2-1.35v.
OK, that would have been my suggestion...
SOC voltage can help too...maybe try 1.05 or 1.1 or 1.125v
Try and get it stable at 2400 and then maybe try for higher frequencies later...