MacBook Air powering down for no apparent reason

I am a newbie trying to learn to repair Apple products. I am stuck on an old MacBook Air. it had fluid damage from previous owner. Once it failed he forgot about it for a number of years.

I have replaced the battery, bought a used and tested logic board, replaced the I/O cable and the Trackpad cable. There were signs of corrosion on all those parts. All else looks good.

When I first got it the machine would not start, charge, or respond in any way. I was able to get it to boot and charge. At first it seemed to work except for the keyboard and trackpad. External devices worked fine. I cleaned the trackpad cable and those started to work. I updated from the original Lion to High Sierra. I got the machine to run for a bout 4 days. Now it shuts down randomly. Full stop. It shuts down quicker as it warms up. Cold start I could get 3 hours. After it starts randomly shutting down the interval gets progressively shorter until the point where I only get a couple of minutes of activity. I got a used but good trackpad cable to eliminate that damage. Still acting the same.

The only other problem is the volume down key does not work. Other than that the computer is fully functional except for the shut down.

I also installed a temperature monitoring program called SMC fan control to see if perhaps the fan was not functioning. It appears to be fine and the chip seems to operate at normal temperatures.

Any ideas out there? A new hobby for me. I love a puzzle.