i've got a Problem with my 360p on a Gigabyte Aorus Pro (non WiFi) Board, which is driving me nuts, because i can't find a reason or a solution for it

The System crashes (Screen freezes for 1-2s then Reboot/rarly complete Freezes) in different Situations:
- While Gaming (Mostly while playing WoW which is hardly using any Ressources, more demanding Games crash less)
- Very Rarly while just browsing the Web
The strangest Part is, the System run sometimes a few weeks without any Problems, then i got a few crashes in a single Day, just to run another Week without Problems, and so on. I can't reproduce the Crashes, running Prime/Videoencoding/Benchmarks for Tests gives no Crashes, everything seems fine. Just to crash a few Days/Weeks later again, which i can't explain.
My Systemsetup:
Processor: AMD Ryzen 3600
CPU Cooler: stock
MoBo: Gigabyte b450 Aorus Pro wifi
Graphics: Sapphire Nitro+ Radeon RX 580
Memory: Corsair Vengeance rgbpro 3200Mhz 8*2gb
PSU: Corsair CV650
And some Harddrives
The Graphic Card and Harddrives come from my old System and worked there without Problems, the other Parts are new.
What i tried yet:
Change the cpu to another 3600 and gpu to rx570 4gb but no luck.
CPU Overclock: With PBO or with manual Overclocks, both the same: crashes. I use normaly manual Overclock, because with PBO the System runs hotter and i got less speed while encoding Videos. Gaming Performance isn't highest priority. I tried to increase/decrease the Voltage, its always the same (except its way to low/high): Benchmarks & System runs fine for weeks, till i got one or more crashes over a few days, till it runs fine again, and so on.
GPU Overclock: Just undervolted the Core, overclocked the HBM. Its the same like it was in my old Rig, and did run there without Problems.
RAM: Ran with XMP and with manual Timings, Memtest says ok, Crashes come regardless of config.
Bios Updates didn't really help, i come from F50 to F51 over the last months and there were no differences in the Crashes. I played with different Settings in the Bios, but nothing helps.
The Temperature is fine (CPU around 75-80C while encoding, <60C while gaming, GPU max 75C, everything else seems fine by me <60C), so think its no Thermal Problem.
I've got some other strange behaivors:
- The Bios resets itself sometimes. Two or three times it even set itself back to the Backup Bios, but that didn't happend since BIOS Version F10. The Resets itself stays, had one last Weekend.
So, currently im completly clueless, i think the Mainboard is the culprit, but i have no Idea if this is true.