I have accepted one of the above posts as the solution, since my machine has become less clogged and I have to seen the "Norton is carrying out a background task while your computer is idle" while I was typing. In fairness to the other authors, all have contributed well to the discussion and helped me think about the problem in new ways.
So far I have:
- set the Idle Time Out to 1 minute. I was set to 10 or 15 minutes I think. This seemed to have made the biggest difference in the dynamics of automated background processes. It appears the Quick Scan is carried out in stages during small gaps of inactivity, but I am not certain. I did not witness those 10-15 minutes of ongoing PC sluggishness of the past. Still I can see regular entries for Quick Scan in the task list (taking around 7 minutes on my system)
- I have disabled MalwareBytes (not uninstalled). I am satisfied that it is not active, I have been watching the process list carefully. I am unsure if it did conflict with 360, as the ill behaviour stopped with setting the idle time out (above) while MBAM was still active. I may do further test and will report back if I have get a conclusive result. I will use MBAM for manual scanning periodically as it was suggested by several gurus.
- I will upgrade to the latest version shortly. I just didn't want to mix in too many fixes; also the new version is likely to have different menus, different terminology etc. so that settings and other things mentioned here may not apply and the issue gets more nebulous.
Thanks again to all.
Matthias