A week later, I've had several days in which the problem did not resurface. However, I THINK I've tied the issue down to the occasions when I have opened multiple streaming tabs in my Opera browser (which is newly-reinstalled for the purpose) and THEN LEFT THE PC ON FOR A FEW HOURS WITHOUT ACTIVITY.
I'm still pretty sure that this only dates from the 2004 Windows Update.
I can easily work around this, but has anyone else noticed such behaviour?