Take a look at the cable connectors on the battery to see if they are corroded or loose. Move the cables around some with the engine running to see if you can duplicate the trouble. Get the battery tested at a parts store. Try a google search for a diagram.

 

Thanks. Battery connections are clean. I should have mentioned, it's a brand-new battery from December. New alternator too - the old bad alternator killed the old battery. I also found out, the headlights do not work when the problem manifests.

I like the idea of jiggling the battery cables to try and reproduce the problem. The temp around here will be consistently above zero in the next few days and I'll try it and report back what I find. I did a Google search for a diagram before posting. I found a 1964 Impala clickbait ad and that's about it. I have a fear that the most inaccessible and hard-to-see connector is my problem.

 

^+1. Battery cables may be corroded under the sheathing and the ends are clean/tight. Btdt, lol.

 

We see this issue on the GMT-900 Trucks (2007-2013) from time to time, and the culprit is usually a bad ground, or bad ground cable. Even though they look ok at the battery connection, its somewhere in that ground line that there is an issue. Might check that out. Also check the main lines feeding the under hood fuse block. Might be loose or chafing. had that issue on my previous 2013, they replaced that entire block under warranty due to melting/heat damage from arcing.

 

Make sure the doors are closed and the keys are not in the ignition at all. I dump mine on the drivers floor or in the console. They migrate out of my pockets when I'm working.
Remove your battery Ground then Hot, clean, lube with protectant, and reinstall the battery side terminal cable connections. Always hook the hot back to the battery first and the ground terminal up last.

I would also look at the G111 main harness ground connection to the transmission.
There are several others scattered through the vehicle but that one is a known problem point.

Your main vehicle Hot connection is #4 on top of the fuse relay block right behind the battery. Worth looking but don't mess with it unless it's loose or very corroded. If it needs to be cleaned remove the battery ground terminal before messing with it.

 

A new battery can still give problems. My 2012 kept holding on to the key after I turned it off. My battery was about 2 months old. The way I figured it out was one time I listened to the radio on accessory for about 15 minutes and the battery died. Nothing would work. I quick charged the battery so it would start, checked the charging system which was good then turned it off. I load tested the battery and it failed miserably. Another battery installed and problem solved.

 

Conduct a BATTERY Test.
Begin with a Fully Charged Battery,
Attach an automotive voltmeter or multimeter to read voltages during test.
Full Battery / Everything OFF.
Voltage: ____
Voltage drop at the instant the Starter is engaged:
Voltage: _
Voltage at Battery while Battery Cranking to Start;
Volts: _
How long Engine needed Starter motor engaged to Start:
Duration: 00:00:00
Lowest voltage shown during Start-up:
Volts: _
Recovery:
Notes: Lowest voltage cranking: _.
Voltage at start: _
Behavior upon starting:
Voltages & RPM's while Engine adjust self upon start:
Volts / rpm's as Computer aids the engine warming-up.
Max Volts achieved and Plant rpm's when Mill settles down self adjusted.
Voltage at Battery post warm-up:
900rpm. Y/N
1K rpm's. Volts. _
12-Hundred: _
15-Hundred: _
2-Grand: _
2500 rpm: (Hold). _

Shine the Battery Posts clean as whistles before testing.
Shine Battery Clamp's Inside diameters assuring A-OK for testing too.