As noted above, fluctuating clock speeds is normal for a desktop environment with assorted WIndows background tasks and services starting/stopping randomly.
If you'd like to induce a constant load, CPU-Z has a 'bench' tab with an option labeled 'stress CPU' which, until stopped, will induce a relatively constant load across all cores; not as heavy a load as Prime95, but, a decent load that will raise temps to about the 'medium-heavy gaming level'... (a 9900K on a mainboard on most z-series mainboards would see all-core turbos of 4.7 GHz for 60-90 seconds or so, with reduced clocks thereafter, unless BIOS and/or Intel XTU tweaked with MCE enabled, power limits/boost duration limits disabled, etc....
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