**On undervolting and custom BIOS settings:**

This machine runs the Prema BIOS mod, and as such, nearly every BIOS option imaginable is available for tweaking.

Our immediate concerns are getting temperatures under control when running with the Intel Core i9 9900k chip on this machine, and ensuring that a proper XMP profile selected and enabled in the BIOS. 

For the correct multiplier settings per-core, under the Overclocking menu (when using the Prema Mod BIOS), stick to these:

Near top in advanced menu (overclocking menu) add in:

Try this for OC settings:

180w for Power limit 1 and 2.
An AVX offset of 3.

For a stable OC, under processor features, disable SGX.

1023 for VR Current Limit (A).
1023 value in Core /IA VR Domain - See the VR Current Limit (A) settings. The Core /IA VR Domain is in another place in the Advanced menu.
Put the same VR Current Limit (1023) in System Agent VR Domain.

For processor clocks:

Base freq: 3.6GHz (unchanged, but can be downclocked via Flex Ratio Override. Leave it as is)
Ring Ratio that is 3x less than the single CPU target Ratio.
Set Ring Ratio to a max of 44 (or 47, but test for stability) and leave it as is.

1-Core Ratio Limit Override: 50
2-Core Ratio Limit Override: 50
3-Core Ratio Limit Override: 49
4-Core Ratio Limit Override: 48
5-Core Ratio Limit Override: 48
6-Core Ratio Limit Override: 47
7-Core Ratio Limit Override: 47
8-Core Ratio Limit Override: 47

CPU Voltage settings:

Mode: Adaptive (default)
Voltage offset (undervolt): 130mv for Voltage Offset.
Offset Prefix to (-) for undervolt.

Other settings:

On power: Enable power efficient P-state and power efficient Turbo boost.

OC menu:

OC feature: Enabled
WDT interface: Enabled
XTU interface: Disabled
IMON Scaling support: Disabled
Vboost enable: Disable
BLCK Aware Adaptive Voltage: Enabled

Boot settings:

Setup Prompt timeout: 1
Bootup NumLock state: On
Legacy Boot Logo: Enabled

Advanced:

ICC/DC Watchdog timer: Disabled
ICC Profile: 0
(Leave the rest untouched)

Advanced/Processor sub-menu: (In overclocking)

Core Max OC Ratio: 50
Core Voltage Mode: Adaptive
Core Extra Turbo Voltage: Untouched
Core Voltage Offset:130
Offset Prefix: -
Core PLL Voltage Offset: 0
Cache Ratio: Untouched
Ring Min OC Ratio: Untouched
AVX Offset:3

Also important: 

Ensure that Ring Down Bin is set to enabled. Reasons: https://www.reddit.com/r/intel/comments/ae958r/whats_the_real_purpose_of_ring_down_bin/ednkx7d/?utm_source=share&utm_medium=web2x

Apply and test.