Thanks. Pop-Ups seem to have stopped by themselves, and no more screen freezes.

But could you say more about:

"There aren't any viruses for Mac, and you running both of those [Norton and could EASILY explain the bad behaviour on your MBP. I would strongly recommend properly UNinstalling both of them. (It's important that you follow their uninstall procedures, not just drag to trash!)."

I know the subject of "no such thing" as (specifically) a virus for a Mac, as opposed to malware in general, has been extensively debated, but what I'm really asking about is how Norton or Sophos anti-virus could be harmful. I've had them for years and am not aware (say again, AWARE) of any damage they've caused.