I am coming to the conclusion that Adblockplus is causing the problem. Over 2 weeks ago I did a clean install on the wife's PC after she suffered repeated problems with IE11 freezing. Since then she's had no recurrence of the problem. Until yesterday, when
she complained about the adverts and I realized I had not re-installed Adblockplus. So I did that (latest 1.5 version) and within an hour she had 4 failures, so I've removed it again.
On my PC I found disabling it did not fix the problem so assumed it was not the culprit. Yesterday morning I completely uninstalled it. I've been having several problems a day, but since the uninstall there have been no more failures.
I'd be interested to know if the other people who've reported this problem are also running Adblockplus. In which case, if you uninstall it, does the problem go away?