Check your CPU cooler installation, otherwise:
"We appreciate the feedback you have provided, and your patience as we investigated this behavior. The reported behavior of the 7th Generation Intel® Core™ i7-7700K Processor, showing momentary temperature changes from the idle temperature, is normal while completing a task (like opening a browser or an application or a program).
In our internal investigation, we did not observe temperature variation outside of the expected behavior and recommended specifications. For processor specifications, please refer to the datasheet.
Most motherboard manufacturers offer customizable fan speed control settings that may allow for smoother transition of fan revolutions per minute (rpm). Please consult your motherboard manufacturer’s manual or website for instructions on how to change default fan speed control settings.
We do not recommend running outside the processor specifications, such as by exceeding processor frequency or voltage specifications, or removing of the integrated heat spreader (sometimes called “de-lidding”). These actions will void the processor warranty.
Support Community Manager"
There is a big thread about these spikes, Thermal sensor issue i7-7700k? so don't worry, nobody cares.