DSP

I would suggest you reset the BIOS to default again, bring your voltage down safely below the default, then bring down your core clocks down to the lowest possible. Do a test. And if it didn't reboot, then repeat the process and increase your clock and voltage little by little until you find your red line. I would suggest a Full Cinebench r23 for the first run and a 2min run for any following test. No need to run full r23 for every single time. For the final test where you think the red line is, do 2 full tests at least. then do a full 1 hour or more of OCCT.
 
These stuff are usually easier and more straighforward with Intel CPUs for me. But should be similar in results with yours. Chances are your processor is unfortunetly just a bad chip and you've got unlucky. So if you can find that red line, then you can stay below it. If you can't find that line then your issue runs deeper.
 
Worth mentioning that this could also be caused by your motherboard failing to correctly deliver to your processor.
 
Maybe someone else could help you better if they have a similar experience with the same board+cpu.

No prob.
It's because the default voltage spec'ed is to guarantee EVERY chip period will function at stock, hence the higher voltage. Some chips are just crap and those take more than good ones do to run at the same speed, that's why they do it.

The way I force my chips to hold a certain speed is to bump up the BCLK by one increment and it holds wherever I set the speed to be, if it's left completely at stock it will change speeds because that's what it's supposed to do by design.

I've PM'ed you a link to a Gamers Nexus vid about how PBO works, it explains it better than I can put into words myself.
Once you understand how it works you'll have an idea of what's going on which helps.

BTW don't forget if you think you've over tweaked it, default settings will get you back into the ballpark of where it should be.

Nothing I could mention that the vid woudn't cover.
I'm not big on running/enabling PBO anyway but that's just personal preference on my part.

For stock speeds and Ryzen, 1.4v's is way too high and I'd have to say it's heating up under load.

I say that because I can run any Ryzen chip I have (Including my 3950x) at or even a bit beyond stock with a little as 1.27v's without issue and some can go lower without issue as well related to voltage used at stock speeds. 

Undervolting itself won't cause alot of problems if you don't overdo it (Go too far down) so what I can suggest is this:

Go into the BIOS and manually set the chip for stock speed and CPU voltage about 1.27v's.
That means stuff like "Offset" for CPU voltage aren't used because if you don't understand what the system is doing when you use offset or you don't figure up the final voltage correctly when using offset, you'll most likely have problems.

Next, do a few CB runs and so on, monitor temps and see where they wind up under load, not at idle using a monitoring utility to see what temps it's reaching.

The way to know if more or less CPU voltage is needed is simple - If it starts crashing and then it does so more quickly each time you try it back off the voltage a little and retest.

If it's more or less taking the same amount of time to crash, maybe a small bump UP in voltage will do but note a bump (Incremental adjustment) means just that - A bump, not a jump in voltage.
If you see higher temps regardless that means to check your thermal paste (TIM) for proper amount/application and make damned sure you didn't leave the plastic film on the bottom of the cooling block by accident when you check it.

I do know though 1.4v's is just too much, even if on water in some cases for Ryzen chips and you're running a larger cored 5900x.

Thanks for the input  
 
I will definitely consider the navi 5700 xt instead, though I did really want to build something with a little rgb bling. Might upgrade the GPU in a month or two once I have the aesthetics done, or vice versa. 
 
I don't actually have the case, however the whole reason I am looking at a new build is because I wanted to shift to a microATX setup which is something I've been wanting to do for a few years now. I found the case for ~$130 in Ireland and think it's worth it if I'm honest. 
 
I wasn't 100% sure on the motherboard so thanks for the recommendation. That board actually looks super nice and would go well with the overall colour scheme I'm thinking of!
 
If I were to pick between either the Ryzen 3000's or a newer GPU, I presume you suggest going for the newer GPU with the 2600? I don't fully understand how bottlenecking works so it'd be great to hear your tips on that front.
 
Thanks!