Quick update. No good news, I'm afraid.
I unplugged the newly-added USB desk lamp, but that didn't make any difference.
I had a long day of work yesterday and everything's all the same - the stutter seems to happen about once every 30 minutes or so, while the crackly and glitchy playback is quite regular - and seems to happen (although I could be wrong) on audio tracks that are from wave files that have been imported into Reaper and converted from 48kHz to the project setting of 44.1kHz.
I haven't had a chance to try WASAPI yet and am hoping the next Windows updates might change things. Otherwise I'm at a loss.
All a bit frustrating this, as I purchased my computer and equipment specifically to start a small podcast production business and wanted to make sure at the outset that the system would be more than enough to handle everything thrown at it.