suspect is CPU or GPU overheat.
please open up your casing and look at the dust on your fan and heat sink.
Just did that to double check, the fan all seems to be pretty clean, awhile ago there seems to be this issue but on a smaller scale eg when I really open a lot of stuff then it'll get shut down so I cleaned all the fan not too long ago.
However for the heat sink I didn't clean it as idk how to, seems like there's more problems to the rig than I expected
While running I just realised the fan below the heatsink wasn't spinning, I manually touched it for abit then it starts to spin. The plate above the heatsink idk what's it called but it's so hot when I touch it I think I could cook an egg & maybe a sausage.
Icic, seems GPU could be the main reasons, you can also try to use your motherboard display/hdmi to test if the video issue happen.
How to do I do this? Sorry not that tech-savvy..
However by luck I could still run games tho, like while I was testing out the 1 ram 2 ram thingy as suggested by y'all. I could still run NBA 2K21 till like 1-2 minute mark then I get that error and shut down again.
Could be psu issues causing the shut down. Can never know unless you swap to another psu. Can also be loose ram or faulty ram. Or can also be a faulty motherboard or gpu. Seems like the problem will occur when the gpu clock starts to ram up so it could be a faulty gpu as well.
I guess the simplest thing you can try now is to reseat your ram or try using a single stick of ram to see if problem persist. Clean off all the dust inside your pc i.e dust filter etc.
Try taking out your r9 290 and use your igpu onboard graphics to see if it fixes the issue. As the gpu clock speeds gets ram up when you start to game or browse videos on the browser a faulty gpu can shut down a pc or cause weird artifact screen issues. (Encountered before).
Can also try disabling this software msi Killer Network Manager as you have said this has pop up before the pc shut down.
If after trying out these easy steps and still cannot solve the problem, then the next step for you is to swap to a working psu, or a working motherboard to fix the problem.
I would say your i7 4790k is still a great rig for gaming and productivity workload. If can should try to get it fix if possible. Can save alot of $$$ if you can get it fix. Or just sell the entire pc and buy a new rig. Trade off is that you will have to spend more money getting a new ryzen rig but good thing is that you don't need to bother fixing this pc.
I would say you should try and get it fix first as buying a new ryzen rig can be expensive. It's your call its your wallet after all.
Yeah I'm suspecting it's PSU based on all the things I google and sometimes the tiaktiaktiak sound and how it's like menses sometimes will shutdown sometimes won't.
True like the other guy Yongkit said, perhaps it could be GPU but depends on my rig's mood sometimes I could run the game 5-10 minutes and now it's just even a youtube video could it down at N-duration depending on its mood..

The day before it was just running fine I could even run multiple instance of it with other stuffs and browser.
Just did the ram thingy and it's fine it could both run on 1 stick alone and it shows up the number of ram on properties just fine, did the Windows Memory Diagnostic and doesn't seem to have any issue with the ram..
Imma try clean up all the dust thingy one more time along with the dust filter etc.. if it still persist then
- Take out r9 and use igpu onboard graphics
- Disable the msi software
- Else it's probably a new PSU and see if it'll fix
-Else it's probably a new motherboard and see if it'll fix
Think it'l be hard to sell the rig tho, where do you suggest? Carousell?
How much would a ryzen rig cost on average? or like the cheapest