My secondary PC randomly reboots

Everything is running stock, even RAM. Sometimes after a week, sometimes after half a day, screen goes black, Windows log in screen appears. It has been happening since I build it, motherboard, graphics card are from a previous PC, where they worked fine, except RAM overclocking on the MB is impossible.
Originally it had Windows 10, fresh install of Windows 11 didn’t change it. I leave my PC on permanently, because of annoying setup and I heard it’s not bad or even good for them. MB went through multiple BIOS versions, now is on stable version from this January.
After ~45 minutes of Heaven 4.0 on Extreme preset and CPU-Z CPU stress test, GPU stabilized at 65°C, CPU 82°C
-Ryzen 5 5600G
-Patriot VIPER Steel 16GB (2x8GB) DDR4 3200 CL16
-Samsung SSD 860 EVO, 2,5" - 500GB
-Fractal Design Ion - 650W

You may have already checked some of these, but here are the first few things I would do (in no particular order):

  1. Use an online wattage calculator to make sure your power supply is supplying sufficient wattage with plenty of headroom.
  2. Re-seat memory. If you have a different set of ram you can temporarily install, do it and see how long it takes to black-screen again
  3. Re-seat GPU and confirm power connections are solid
  4. Confirm that your mobo battery is not discharged (battery type is usually cr-2032 or similar)
  5. Check mobo utilities to confirm fans are operating, voltages are normal, overclocking options are off
  6. Check mobo attach points, stand-offs, contacts with chassis (ie, make sure no possible short circuit points, or unintended ground points that could be sparking off of static buildup on the case)

Maybe something in this list of things to try will turn up a clue?

Could be your power supply - i also had this issue once - was my Psu
If you have a 2nd psu around 650/700watts you could try that

Says your graphics needs 550watts
Cpu around 65 watts
So when the psu spikes it might not be suffecient with 650watts

If you got access to a 700/800watt power supply i would try that first

Issues like this are no fun. I waited to see if it happens again and the second problem happened. The PC froze. I reseated RAM, GPU, it has a riser, removed BIOS battery, waited, put in a new one, cleaned and inspected the PC.
Now I wait.

Maybe driver conflicts that only arise when a combination of things, Windows protection and drivers all argue over various other things?

82C temp on the CPU is a concern too. Not uncommon these days but still a concern.

I just noticed you said it’s running stock and at 82C, how long has it been since the CPU has been repasted and what is the heatsink?

I had issues like that when I first put together the rig I’m working on. Turns out I had the pump controller for my AIO plugged into the wrong header on the motherboard so the CPU was constantly overheating. Its always a challenge doing a full build like that because all the components are from differing manufacturers and none of their instructions line up perfectly.