Well, I think things are finally running smoothly again! (Though I guess I’ll only know for sure after a few more photo shoots.) I followed your instructions, @buddingphotographer, and backed up my catalogue the way you said, optimised LR, shut down the program, deleted the preview cache, reopened LR, and went on vacation (well, not quite – I just went out to see how our new ram was faring). But that didn’t fix it, so I went on to the next step and purged the Camera Raw cache file. That seems to have corrected the problem! I was able to reimport my “problem” files with no difficulty.
Thank you so much for you help! I was getting rather worried that my card was corrupted or something else irreparable. Thanks!