Hi Steve,
Thanks for writing. My memory is that the Preview smooth text option was selected originally, and that yesterday I unselected it as part of my troubleshooting. Then today I reselected it on your advice, and now some of the PDFs display correctly. The PDFs all display correctly in Adobe Reader.
However, I can still replicate the issue in some PDFs. There are individual pages where upon scrolling the text gets very choppy in Preview but not in the other apps.