CPU frequency down to 2.1 GHz with only 66 °C on Inspiron 15 7567

Hello

I am using  Inspiron 15 7567 with i7 1TB HHD+256GB SSD with the up-to-date BIOS 1.0.4.

I use the default settings in Intel Extreme Tuning Utility and start CPU stress test and FurMark to stress test it. From the diagram below. At the 3rd minute on white line. The CPU temperature hit the 80 °C and suddenly frequency down to 2.1Ghz and never raise up even it is only 66 °C.