Michael,
I see the problem in the code now. It only preloads when you're searching metadata. A filename only search doesn't preload and it's still possible that the images that haven't been seen don't yet have their filenames either.
A workaround until I fix the problem would be to set the Metadata checkbox, and then uncheck all of the fields (so that they won't end up being considered) and then do the Find. It should then preload and proceed with the finding.
-Kirk