First off, if the CPU is overheating at idle then you have something seriously wrong elsewhere.
Second, while Intel will never recommend overclocking it doesn't mean you cannot overclock or that it is even unsafe to do so. This is just Intel's way of having no responsibility for damaged that MIGHT be caused by overclocking.
Lastly, that Ram speed should be fine, but remember that ram that is higher than the CPU recommended maximum will overclock the Integrated Memory Controller. So while this is normally of little concern it can affect temps/stability if you are already on the edge of either.
This message was posted on behalf of Intel Corporation
Thank you for contacting the Intel community.
The thermal sensor issue you can see more information at the following link:
Basically 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 Intel® Core™ i7-7700K Processor Product Specifications.
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.