I think you might have got it right-on! The odd thing was, I would have a check-engine light but no code; the reader would also indicate that the current status of the light was Off, but if I choose Reset, it turned the light off. I stopped by AutoZone, and their reader showed a PO133 code (may not remember it precisely) but it indicated an oxygen sensor switching slowly, which sounded like a vacuum leak, possibly. Indeed I found a deteriorated little rubber elbow on a vacuum line, so maybe that was it, or part of it. I did get a code soon afterward (which of course my reader said No Code!, but I deleted the code and it hasn't popped back up. I will continue to search for other vacuum leaks, but also I will search for a new code reader!

Thanks for your thoughts!