I did memory tests at 3000/3200 MHz for either set of RAM, no issues, for months I did various things, gaming, mining, rendering, etc, PC's been on pretty much 24/7.
8GB @ 3200 MHz for couple of months, then I needed more RAM, so I upgraded to TridentZ 16GB @ 3000 MHz, ran fine for another couple of months, doing same things I mentioned, then system started freezing when I do anything CPU demanding, so I dropped it down to default/2133 MHz, which was stable for a bit, then upped it to 2933 MHz to gain a bit more performance, still stable and left it at that ever since, but that's after I swapped back to 8GB RAM because RGB lights on 16GB RAM was annoying me, no way to control them on Linux or in BIOS/UEFI, I've only ran Linux (Xubuntu 16.04 then 18.04, to be specific) exclusively on this system.
If I was in your shoes, the first thing I would do would be to completely forget the fact that it did work at some point and tackle the issue as if this was a brand new system, going through all options 1-by-1 with a clean-slate.
I don't know much about running Linux (always wanted to try, though..) but I did a quick google search last night, for your board, and I found a forum post talking about a specific option you have to enable on that board's BIOS, to allow for higher RAM Frequencies (+3000Mhz)...thing is, I can't remember what it was called :/ sorry.
The fact it did work for months and then stopped, does seem strange but I recommend starting from scratch and going through every single setting to ensure nothing got reset, that could be stopping you from using stock RAM speeds.
Just to cover all bases, what about your PSU? Is it a quality one? If not, have you made sure you haven't lost a significant amount of voltage on the 3.3v 5v and 12v rails? (are they reading 3.28v, 4.8v and 11.7v respetively {or any lower than that?}), obviously there will come a limit when you won't be booting at all (or get serious instability issues) if it was lower than what I quoted above, but it's worth checking that variable in the equation, just to be sure nothing out of the ordinary isn't skewing your attemps to set 3000-3200Mhz RAM Frequencies.
Also, what I am most curious about is: what would happend if you tried increasing the voltage on your RAM to improve stability?
Good RAM sticks will run at a rated 1.2-1.25v (check your RAM specs to be sure), but for DDR4 in general, I understand that it can run up to 1.4-1.45v and you don't realliy want to go above that (not like DDR3 which could run up to 1.5-1.65v (I definitely wouldn't take it above 1.4v myself).
So, if you're trying to run 3200Mhz at 1.2v, maybe increasing the RAM voltage to 1.25 or 1.3v may help in stability, perhaps.
Also, I know that for Ryzen, increasing the VCCIO voltage can help with RAM stability (be careful with this one, though, and raise this very-very slowly IF you have the option to do so...normally it starts at 0.9v and you don't really want to go about 1.2v max).
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So, my RAM is 2400Mhz CL16, 1.2v // and I am running it at 2933Mhz CL18, 1.35v and VCCIO at 1.1-1.125v]
Everything's on auto, except for CPU, which is set to 3.8 GHz @ 1.325v (3.8 GHz seems to be the ceiling for my CPU, anything higher and it won't boot, no matter what voltage I give it, but that's not the issue here, unless it's somehow related and I'm missing something, just curious, what settings are you using for 4 GHz?).
Interesting because I can hit 4Ghz with 1.32v, but I have disable certain power options (the auto-turbo one, disabled the Global C-state), and I have LLC on High.
If you do have the option for controlling LLC on that board, by all means, post what you got, see if we can't tweak it further.
Ryzen can take safe voltages up to 1.4v but I avoid going over 1.38 to be safe (I am on the stock cooler for that CPU, so for stress tests at 1.32v/1.33v I am hitting 75-76C max, and gaming is under 60C.
I also have a case with 6 Fans for maximum, and optimal, air-flow.
Here is one of my UserBenchmark scores: https://userbenchmark.com/UserRun/13559136
Motherboard is ASRock Fatal1ty AB350 Gaming-ITX/ac, haven't done any BIOS update, it's running at whatever it came with, but it's one of the versions without support for Ryzen 2xxx series.
With regards to your board,
if someone can correct/educate me on this... I thought it strange that it doesn't support Ryzen 2xxx series...oh well.
Back to the issue at hand: Definitely try to see if a small bump in RAM voltage (+0.05v) and a tiny bump in VCCIO voltage (but not going above 1.1-1.15v total VCCIO), may help with stability.
The other option would be to try to loosen the timings the bit, from their stock timings, but if you're not used to this, I wouldn't recommend it without a thourough reading session of what the timings actually mean..so you get an idea of how to set them.
Hope this helps.
