Ok, I think I get the structure of those two files, as I can see that the syslog is updated constantly while the diagnostics hasn't changed since Feb 23. I will continue to upload the syslog to pass along the most recent info as we progress with troubleshooting.
I'm fairly sure that cooling and power are reliable. This hardware used to be my main rig, I was running the same hardware with different SSD/HDDs plus a GTX1080. I was slightly overclocking the CPU to 4.7GHz from 4.4, on air using a BeQuiet Dark Rock 3 which has a 250W TDP rating. Under stress test I was hitting high 80s or maybe 90 degrees but it was stable when running Win 10.
The PSU is a CM600X Corsair from 2014. It is 80+ Bronze certified, though it certainly isn't top of the line, and it is 5-6 years old now. It performed well while overclocking the old rig, and I'm not asking nearly as much from it in the current setup. I have 3 fans moving air in the current Full ATX case, and it was recently dusted. CPU temp in memtest is currently bouncing between 55-60, I have no frame of reference, but that seems normal for a light load. I think it was low 50s when running the single core test. I don't remember noticing high temps when stressing the server (running a W10 VM and 1 Plex stream is probably the hardest I pushed it since setup), but I can't say for certain.
That's all the relevant info I can think of re: power and cooling, but I'll definitely keep a closer eye on temperature data moving forward.