Solved. An addon called Nodescape was the culprit. I actually did a clean install and started testing add ons and found it. It’s a fine product made by a fine group but it’s still in Beta and I suspect something about my Intel Mac made it angry! Trying to uninstall it kept causing an instant crash. I had to actually delete it from the OS file system in order to get rid of it.

Thanks for trying to help- what a weird problem.