Sounds like PHP might be timing out before it finishes making all of your thumbnails. If it starts over each time, it might behave exactly as you describe. Either that or a filename in that directory is causing problems for the script.
My suggestion, just to try: split those images into a couple different directories, and add the directories separately.
This problem shouldn't exist in 1.1 because it builds thumbnails in a different way, and you can create thumbnails individually, too.
I don't want to say to you "works for me", but it does. Perhaps are there odd characters in your captions? Try deleting anything "odd" like punctuation or symbols.
I'm at a loss as to what could cause this. Maybe if you sent a snapshot of your exhibit table? (ringmaster [at] midnightcircus [dot] com) Send a warning email first so I can add you to my spam whitelist, otherwise your attachment will be deleted and your email incinerated. :)