As far as I can tell, what you are seeing is completely normal and expected. Significant changes in processor temperature can occur very rapidly. While you might think your system is idle, Windows 10 has many background tasks that run when the system is detected to be idle and thus seeing changes in processor loading and temperature is normal and expected. Finally, if your temperature under heavy load only goes to the 80-90c range, then your overall cooling solution is working properly and the processor kept happy (at least thermally).
Hope this helps,
Hi, I have an i3 9100f and its temperature seems high too. I did a load test using Intel Processor Diagnostic Tool and the temp goes to 81 celsius.
Do you tried to change the cooler box? I'm thinking about doing and I'd like to listen someone's experience with this processor.
My computer seems quite like yours.