" that was the first thing I tried. "
Does that mean you have tried ALL the suggestions or just some of the suggestions
Giving the benefit of the doubt that ALL have been tried.
1 - There could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.
2 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.
3 - Download the Application Etrecheck directly from the Developer.
This is a Diagnostic Tool that makes no changes to the computer.
It makes a coherent and readable inventory of both the Hardware and Software used on the computer
The application is free or paid from added features.
The Report will Not Reveal Any Personal Information.
Post back the Full Report - copy and paste - >>>> using the Additional Text Icon ( 3rd Icon to last ) <<<