I bought a desktop from HP with an I7 8700 cpu. It has shut down 6 times (3 times in one weekend, often overnight) with a message System Temp (90D) Thermal Shutdown. I run all 6 cores at 100% 24/7. CPUID HWMonitor shows the cores running at between 83C and 87C which is below the 100C max temp Intel shows for this cpu. Would that indicate an intermittent cpu fan/cooling problem?
This thermal shutdown is occurring as a result of an ACPI rule set up by the BIOS. I would check in the BIOS to see if you have control over the shutdown temperature being enforced by this rule. To avoid these shutdowns, you could raise this temperature to somewhere in the mid- to high-90's.
Now, if you are keeping this system that busy, it is no surprise that the temperatures are in this range. From a fan speed control standpoint, your response curve should ensure that, if the temperature is above your desktop processor's Tcontrol temperature, the processor fan/blower/pump is at full speed (100% duty cycle). If you do not have a tool that can display your processor's Tcontrol temperature, it is typically in the low-80's, so you could use an estimate of, say, 83c.
Hope this helps,