Disk Utility "First Aid" says "Problems were found with the partition map which might prevent booting" after clean install
I created a bootable external HD running 10.11.6 and I can boot from it. I also erased my internal and reinstalled 10.11.6 on my internal SSD. I booted to the external after selecting it as Startup Disk in Sys Prefs, and ran Disk Utility on the internal. Afterward, I no longer get the message "Problems were found with the partition map which might prevent booting." Phenomenon of Mac momentarily "going into neutral" described above also appears resolved (based on 30 minutes of use, anyway). Soon I will boot to external and rerun Disk Util to recheck for errors on internal. But so far, seems good.