8700k Underclocking during stress test, cooling not an issue?

[EightyHD]

So here is the short version:
When doing some (not ALL) stress tests, my i7-8700k @ 5.1GHz will underclock to under 4GHz, it does NOT happen while gaming, rendering, etc. Cooling is not a problem as it rarely hits 75 celsius, and low 30s idle. AVX instruction set is at 0.

Specification List:

i7-8700k @ 5.1GHz
Corsair H115i Pro
Corsair VENGEANCE RGB PRO 32GB 3000MHz
MSI Z370 Gaming Plus ATX LGA1151
Gigabyte GTX 1070 G1

Cry baby version:
What's poppin guys? I have a question that I am struggling to figure out on my own.
Back in April, I jumped from a 4790k to an 8700k, and I am very happy I made the switch. This thing destroys compared to my (iconic) ex-processor. It will be dearly missed.
Anyways, I am familiar with overclocking, at least on Haswell chips, and was figuring that this chip will be a breeze, and it was. However, once I overclocked to a decent 5.1GHz I noticed my scores were a little below my STOCK speeds. This very well pissed me off. After investigating further I noticed that when running some stress tests, my clock speeds will drop all the way down to 3.5GHz, and jump in between there. My last processor did not do this, and I know that you guys were jumping to the conclusion of poor cooling and high temps. My temps rarely hit 75 celsius when stressing the CPU (I am using an H115i Pro with push pull config) so it isn't even close to hitting the tJmax. I even have AVX instruction set to 0, which has been said to help problems like this but sadly it hasn't. I know stress tests really don't matter, and it doesn't happen when I am rendering or gaming, but I like to call myself an enthusiast (lol) and just KNOWING that my computer can hit a higher score but is limiting itself for what seems to be no reason really grinds my gears.

The Big Question:

Has anybody else had a similar problem and figured it out? Or any other suggestions, please let me know!

 

[bignastyid]

Best guess: VRM throttling. That board has fairly basic VRM setup(not good for overclocking) so it will struggle with the big OCs like what you are trying to do.

 

[mdd1963]

Your mainboard's BIOS might have some power-limiting feature enabled somewhere....

Also, double check 'Performance vs. Balanced' in Windows power plan, etc....

With MCE-on, the default all-core turbo of 4.7 GHz with all 6 cores might often be superior to a single core turbo of 5.1 GHz, despite 'sounding less impressive'...and might be more sustainable within power/thermal limits.

You'll have to decide if running a constant 5.0 GHz, running 150 watts consumed, even when doing essentially nothing, surfing Youtube, is worth it over default/MCE modes that thottle up only under a system load.....

If you wish to disable the MB's safety features, might need to search for specific BIOS settings to disable thermal/power limits for your specific MB, I'll not link to it in light of it potentially damaging something all for the magic 5.0 GHz barrier's sake...