https://pcpartpicker.com/list/FxT3tg" rel="nofollow - PCPartPicker part list / https://pcpartpicker.com/list/FxT3tg/by_merchant/" rel="nofollow - Price breakdown by merchant

CPU: https://pcpartpicker.com/product/Mr2rxr/intel-core-i5-8600k-36ghz-6-core-processor-bx80684i58600k" rel="nofollow - Intel - Core i5-8600K 3.6GHz 6-Core Processor   ($238.99 @ Amazon) 

CPU Cooler: https://pcpartpicker.com/product/RH2rxr/corsair-cpu-cooler-cw9060027ww" rel="nofollow - Corsair - H115i 104.7 CFM Liquid CPU Cooler   ($134.51 @ Amazon) 

Motherboard: https://pcpartpicker.com/product/9pZ2FT/asrock-fatal1ty-z370-gaming-k6-atx-lga1151-motherboard-fatal1ty-z370-gaming-k6" rel="nofollow - ASRock - Fatal1ty Z370 Gaming K6 ATX LGA1151 Motherboard   ($181.98 @ Newegg) 

Memory: https://pcpartpicker.com/product/qjM323/gskill-trident-z-rgb-16gb-2-x-8gb-ddr3-3000-memory-f4-3000c16d-16gtzr" rel="nofollow - G.Skill - Trident Z RGB 16GB (2 x 8GB) DDR4-3000 Memory   ($199.99 @ Newegg) 

Storage: https://pcpartpicker.com/product/ft8j4D/crucial-mx500-500gb-25-solid-state-drive-ct500mx500ssd1" rel="nofollow - Crucial - MX500 500GB 2.5" Solid State Drive   ($124.99 @ Amazon) 

Storage: https://pcpartpicker.com/product/XtjG3C/western-digital-internal-hard-drive-wd2003fzex" rel="nofollow - Western Digital - BLACK SERIES 2TB 3.5" 7200RPM Internal Hard Drive   ($109.99 @ Amazon) 

Video Card: https://pcpartpicker.com/product/FdH48d/nvidia-geforce-gtx-1080-8gb-founders-edition-video-card-9001g4132500001" rel="nofollow - NVIDIA - GeForce GTX 1080 8GB Founders Edition Video Card   ($766.00 @ Amazon) 

Case: https://pcpartpicker.com/product/2mQRsY/phanteks-enthoo-pro-m-tempered-glass-atx-mid-tower-case-ph-es515ptg_bk" rel="nofollow - Phanteks - Enthoo Pro M Tempered Glass (Black) ATX Mid Tower Case   ($89.99 @ Amazon) 

Power Supply: https://pcpartpicker.com/product/jWFXsY/seasonic-focus-plus-gold-850w-80-gold-certified-fully-modular-atx-power-supply-ssr-850fx" rel="nofollow - SeaSonic - FOCUS Plus Gold 850W 80+ Gold Certified Fully-Modular ATX Power Supply   ($92.99 @ Newegg) 

Operating System: https://pcpartpicker.com/product/wtgPxr/microsoft-os-kw900140" rel="nofollow - Microsoft - Windows 10 Home OEM 64-bit   ($92.99 @ Amazon) 

Total: $2032.42

Generated by PCPartPicker 2018-04-18 09:56 EDT-0400 

Since I put everything together I have been getting IRQL BSOD errors which have made my machine basically unusable. At first I was able to use windows for an extended period of time before a BSOD, but it eventually degraded to a BSOD as soon as windows launched. I had been troubleshooting this issue for the last week without much luck. Two days ago I noticed that my asrock was supplying 1.136 volts to my CPU (this is with the latest BIOS and all stock settings). I'm not an overclocker but that voltage seemed to be a bit low to me so I consulted reddit for an answer. Some people seemed to agree, and suggested I try manually upping the voltage. Last night I set the voltage to 1.2 and now everything seems to be running stable!

My question is this... what gives? Is this a bad CPU? A bad motherboard? A bad PSU? I'm an experienced builder and have never had to manually tweak the voltage to get a stock CPU to run stable. Does anyone have any insights?

I am going to assume you have already cleared CMOS (likely more than once in a week of troubleshooting). The issue is present even at factory defaults? If you have not already, it may be worth pulling the CPU, cleaning it's contacts and reinstalling it. If something non conductive coated the pins responsible for the CPUs internal voltage sensors etc it could also cause the issue.  

We haven't seen any other instances of your issue here on the forums so I would be willing to bet it isn't the BIOS. If cleaning and reinstalling the CPU doesn't work I would RMA the it, particularly because the issue got worse as time went on. The CPU's voltage regulation circuits could be degrading. If this were a system out of warranty I would suggest just using it with the manual voltage minimum but if you can return it, you should. 

Good luck and keep us posted