So I've had these issues, I initially thought it was the PSU due to not enough power 500w, however I think all the symptoms are pointing to the mobo maybe?
USB 2.0 failure at completely random intervals. (Only have 2 for mouse and keyboard)
Ram slot failures of 3 (except slot 4)
It doesn't crash.
Slows down randomly (however this issue can only be recreated by my missus, I don't know why it never happens when I use it and I do a lot more heavy things)
If it was the PSU it would only fail under max load right?
*Note I was cheap and used a case+psu combo which I know is a big no no.
Mortar Artic B350
Ryzen 5 1400
8gb Ram (was 16 before failure)
GTX 6gb 1060
2x 1tb WD green (they were free)
1x 256gb Samsung evo 960 Nvme
Cougar ST-500 PSU