Mac keeps logging me out

My Mac Pro is logging me out about every hour or so of active use with multiple windows/programs open. I believe this is the same issue as this: Why does my Mac keep logging me out? and this: Re: Mac Os Sierra keep logging out !! but both of those forums are from 2016 and discussing Sierra. I'm on High Sierra. I am using Chrome, and turned off hardware acceleration, which appeared to help, but then the problem returned (with that setting still disabled). I do not use Rapport and have no file called com.apple.universalplist, and do not have auto-log out after X minutes of inactivity enabled, which are common answers suggested in those older forums.

Any thoughts anyone? It's a pretty awful time being in the middle of various tasks to have every app quit all at once with no warning. Doesn't seem to be connected to full-screen apps and I'm not watching YouTube when it happens (other commonly cited triggers).

Mac Pro (Mid 2010)

Processor: 2.8 GHz Quad-Core Intel Xeon

RAM: 16 GB 1066 MHz DDR3

OS: 10.13.6