when the cpu idles and you have a balanced power setting the frequency fluctuates to save energy, if you want it to ramp up to max frequency all the time, set the power setting to High performance.
Hi, you can download hwinfo portable from its page: https://www.hwinfo.com/download/ and run the "only sensor option".
Then go play, benchamrk or whatever you do with your PC while hwinfo is on the backgound, if your CPU hit 5GHz during that time, it will show on the MAX cloumn