Another Nexus 5 w/Android 6.0.1 with excessive Battery Drain/Random Reboots

Apologies for the long initial post.

Google/LG Nexus 5
Android 6.0.1
Security Patch Level Oct 5, 2016
Kernel 3.4.0-g99666f7
Build M4830X
Not now nor has it ever been rooted
No custom roms or any other non-factory configs

I have two (2) Nexus 5's with the same specs as above purchased 6 months apart in 2014. The oldest one has no issues. The newer of the two has started experiencing an excessive battery drain and/or stability issue this November. I've been searching, reading, following other troubleshooting suggestions but this situation isn't adding up to me so I'm posting for assistance. I won't take offense to anyone posting a link to another thread telling me the search tool is my friend. I haven't been able to find the correct answer on my own.

Problem: Phone will perform a hard power off while in use after a very short time with plenty (well over 25%) power remaining. No low power warning or anything. Is there any fix for this as it makes the phone essentially unusable?

Issue Details: The phone would be used maybe 10-20 minutes then it would simply shut off. No low power warnings or any kind of grace period. Attempts to power it back up would fail. Let the phone sit 5+ minutes and sometimes it would power up and always with some random power, 15%, 87%, 42%, whatever it apparently felt.

Troubleshooting: Wiped the data cache 3x, factory reset of phone 3x times. Running minimal secondary apps (zero games, utilities, etc). Performed the battery memory pcb reset whatever it's called process of full charge, full discharge, full charge, full discharge, full charge with the OEM LG cable and charger to ensure the battery meter is functioning correctly. Moved the SIM out of the device into the older Nexus and a different but inactive SIM into this one to see if the issue followed it, it did not follow the SIM. Ran the phone on Airplane Mode but with Wifi enabled. Have ensured wifi scanning is off as well as various other basic battery saving techniques I could find. Extra case does not cover power buttons at all but I removed it as well as followed some power button "cleaning" techniques out there to ensure it's not stuck. I have not opened the phone case at all. Power history shows nothing out of the ordinary, just a gradual decline in power followed by an abrupt drop when it shuts off, then after boot more gradual decline.

Current Status: Battery appears to be reporting more consistently now, the apparent massive quick drain is gone but the phone will still shut off. I charged it fully Sunday night, unplugged it and left it idle overnight and into Monday AM. Decided to read some news on it, power reported along the lines of 67% and within 5 minutes it powered off in my hands. Did this test again 11/22 am. Full charge overnight, set it on my desk while working for approximately 3 hours while it was in airplane mode but with wifi enabled. Lit it up, showed 96% power so I started surfing google news feed. I estimate within 3 minutes the phone powered off while in use. Attempted to restart the phone but it would power off before the word ANDROID would appear but after the multi-color circles show during boot up. Left it for 20 minutes came back, powered up fine, reporting 86% power.