It's funny, yesterday I tried it again and it worked for about an hour and then went to the cover art stop.
I have tried changing themes, using different combinations of scraping, and all the standard skins. Tried creating my own theme. None of those seem to work. At first I thought it was a glitch with recycling through previous viewed pictures, but then I realized that wasn't it. It may be certain artists and not others or even albums and how they're configured with tag or file structure. I listened to George Jones' Greatest Hits and it did it there, then did Gary Puckett and it did not. Does that make sense? Maybe MC21 isn't a country fan, but likes 60's easy listening.
I know it isn't multiple artists albums vs. single artists albums. Tried albums tagged as podcasts or mix albums, no correlation.
Bill Wither's "Ain't No Sunshine" did it every time. So I looked at the album work (FLAC file) and there was the cover in the file and a cover in the folder.jpg file. The folder.jpg file was a different cover art. I deleted the second and it didn't do it. Could this be the problem? I have over 10,000 files and it would be time consuming to see how many had different covers in the files and folders.
Edit: After some more experimenting, deleting the second artwork in the "folder.jpg" file has cured it every time. Artwork is saved in the Flac file and evidently causes a conflict in MC21 with the folder.jpg file in the slideshow. I have kept the artwork tagged in the file and deleted the folder file and it appears to fix the problem. I have files that contain more than one cover art in the FLAC file tag, but they don't seem to be affected.