Is that rear NVME drive Gen 4 ?
They get hot enough without doing that to it in an A4 SFX hehe.
Regardless, I'd either remove it completely from the rear slot and either replace it with a Gen 3 drive which run much cooler anyway, or just use a sata SSD.
Some further elimination required here with testing as well:
CPU first with Cinebench R23 for a good 10 mins looping.
Then run TestMem 5 or Karhu ram test and see if the ram craps out in testing.
You don't need to do this for hours and hours, it's just quick and dirty fault finding.
If the CPU and ram don't crap out in those tests, we've eliminated those.
The rear NVME drive could also be causing this, remove and test without it.
See how you get on and report back 
