You should be using Ryzen Master to monitor temperatures and such, though both will display the incorrect voltage if you manually set it.
85*C under heavy load is to be expected with the stock cooler and Turbo enabled, it's AMD's way of ensuring maximum performance is safely extracted, as the thermal maximum is 95*C. Idle temperature is very high, but then again if you're measuring the idle temperature while using an internet browser, especially Chrome, elevated temperatures are to be expected.
Prime95 and Ryzen processors don't mix well, and Prime95, along with all other artificial power virus tests, have long been known to cause false positives for instability due to the way they work, only made worse when it started using AVX. I've seen mine fail it even at stock settings on my 1800X and now on my 3700X and I have a much more powerful than stock cooler, but I've also hammered it with AIDA64 cache tests and had it grinding away for 48 hours straight on video encoding without a hiccup with my overclock and undervolted settings (4.3ghz 1.2v) which I run every day.
That's good to know, but how should I address the stuttering and blackouts problems? It seems that using the A-XMP profile does away with the screen blackouts, but the stutter is ever-present, just not as frequent.
Again, apologies for the late response.
While you had your stock cooler on, were there times when your PC stuttered, or the temps ever touched 100°C?
I tried undervolting VCore but going below 1.35V always caused PC crashes owing to memory controller problems.
It's also obvious that affecting my ambient via air conditioning helps the CPU temps as well, dipping by as much as 7-8°C. A new cooler might be needed.