Wow, thanks for all the pointers!
No nasty clunking HD sounds, and nothing really bad in the Event Viewer. mouser's comment about CPU usage (when those problems occur CPU usage sometimes seems to be at 100%, sometimes not - I need to monitor this more) reminded me of something I *did* change some time ago: I set my PC's (it's a laptop) energy options from "energy saving" to "balanced" in hope of a better system performance. I now reversed that back to "energy saving", and the PC behaves way more smoothly. Interesting (and a bit strange)... I'll see what will happen next.