dual screen resolution setting is reset after logout, reboot or sleep On a mac mini running OSX 10.11.6, if I set dual screen resolution to 1920x1080, it resets to the default 800x600 if the computer is reset (or if slideshow mode on powerpoint is activated), or if the user logs out, or if the computer is awoken from sleep. I have tried resetting the nvram, to no avail. 

How do I make this setting persistent? Note: I need the setting to be persistent while in guest mode. I have three other mac minis with identical configurations whose dual screen settings are persistent, even in guest mode.
Try setting your resolution to the desired level, then simply logging out and logging back in. Does the issue persist?

Have you tested in Safe Boot? Please do, it'll tell us a lot.