It’s not resolved completely, see my post here Tracking thread for Norton 25.9 / 25.10 / 25.11 / 25.12 releases - #42 by user10572
But, partly thanks to @Frank_V1 ‘s post above, in the middle of the night I had a brainwave.
a) Why does Frank_V1 get lots of advertising when I don’t?
b) Why isn’t everyone suffering from this problem?
c) What’s the common factor between my three PCs that all suffer from this problem?
Answer - network ad-blocker!
When this problem is apparent, with Norton 360 sitting in the background in the system tray, it is making calls every couple of seconds to ipm.avcdn.net, which my ad blocker blocks. According to Norton and NextDNS | MalwareTips Forums this site is responsible for in-product messaging, promos, etc…
As soon as I open Norton 360 to the desktop, the high cpu, flashing cursor and calls to ipm.avcdn.net stop.
I’m waiting for a call from Norton second line support today or tomorrow, I’m going to be interested what they say. I expect the answer will be to whitelist the above site as it’s “essential” to the operation of the tool. Let’s see!