My CPU Reverts To 3.6Ghz Under Load

Apr 25, 2018
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Hello, i have an i5 6600k paired with a gigabyte B150-HD3 motherboard and a hyper 212 evo led turbo.
I was able to change the multiplier to 39 in the bios from the initial 35, but when i ran the cpu benchmarks they dont dont seem to have changed at all !
Upon further inspection i noticed that under little load ( 10% or so ) my cpu reached the 3.9 ghz. But under full load it it reverts back to 3.6Ghz
It can't be thermal throttle since the max temperature was 59c
I'm new to overclocking so I can't figure this out.
Keep in mind i haven't touched voltage yet, its in auto as far as i can recall